Let us start from the general remark concerning historical accuracy or authenticity in fiction, and then we will move to the matter at hand. Of course, I wholeheartedly suggest you to look up any topic that might have interested you in this subreddit's FAQ for more in-depth information.

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Introduction or why the fantasy does nor need to be historically accurate

When speaking about the historical accuracy in fiction one must ask oneself, what kind of fiction is it. If it is a work from a historical fiction genre, then it must maintain historical authenticity at the very best, with more or less accuracy thrown to the mix depending on whether or not the work will portray real people and events in the purely historical setting (this differs from alternative history, where the events are deliberately different, but that's another matter). I'm differentiating between 'accuracy' and 'authenticity', as the former is depiction of the certain elements as they happened according to the actual historical sources (e.g. historically attested people doing things they actually did in a given place and time, such a Richard I dying of sepsis on 6th April 1199 after being shot with a crossbow during the siege of Chalus-Chabrol), while the latter is a depiction of fictitious persons and events that, although completely non-existent in history, could have well existed as their character was in line with the characteristics of the given time and place.

Having said that, I would like to note that I generally recommend people who want to write complete fiction that does not bear any relation to history, such as fantasy or an alternative historical fiction that is heavily divergent from known history that they do not bother much with real history. As much as it is counterintuitive, it only makes things less plausible to anyone who actually known history because such a person will immediately notice lack of cohesion between elements of the depicted world. For example, one might painstakingly describe historically accurate feudal relations only for a person well-versed in history noticing that it makes very little sense for the feudalism to actually develop in a given territory. In other words, while gathering material for a fantasy or similar work of literature, it pays to learn not necessarily what happened, but why it happened in the first place. Especially if this is anything related to society or economy. Of course, it is also understandable that the author might write with a specific audience and if it happens to be composed chiefly of people who have a rudimentary knowledge of historical trivia, but not of historical processes, a superficial authenticity might be required to keep such people interested in the given story.

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Medieval armies or how to be and not to be present on the battlefield

Now, let's move to the actual questions. First and foremost, let us discuss the term 'peasant' and 'sent to war', because it is very possible that the entire question is based on the false premise. If by the said inquiry we are to understand the situation that the peasants are conscripted to the medieval army (and with the usage of the term 'medieval' I assume Western Europe between 6th and 15th century)or a world based on this particular region and space) then the answer would be rather simple, as no such thing happened then and there. In general, peasants were not conscripted, definitely not en masse and not if the actual combatants could prevent it which means most of the time. European armies in early and high Middle Ages consisted largely of the 'knights' and their retinues, i.e. people who were granted land by their senior and could have reaped all the benefits from it in exchange of military service to a prescribed standard, usually meaning answering the call to arms armed, armoured and mounted wither alone or with a retinue of a size dependent on the size of the land held. In this vein, one could have said that there were no peasants on the medieval battlefield. Sure, in some regions peasants were possessing arms, they might have been required of military service or even undergoing training, but they were generally people more valuable as a workforce than as a military and were used for warfare only in dire situations (read: last-ditch defense of land). If anything, one of the reasons for commoners accepting the rule of the feudal lords was the lack of necessity to fight, as the defense of their and was the obligation taken of the landowner.

But there is a very important caveat. By saying 'peasant' we usually mean a commoner who is obviously not a noble, but also does not belong to clergy not is a burgher living in a medieval city and enjoying burgher rights (but also bearing associated burdens, such as paying for weapons and armor and defending the city if necessary). In this vein, a peasant is anyone not belonging to any of the other estates (nobles, clergy, burghers) but also living in a feudal structure of the land ownership, unlike 'loose people' who did not own anyone anything but also were not protected by anyone, such as vagabonds, brigands, beggars, Gypsies etc. And such people could have and did appear on battlefields, often in quite a numbers. But doesn't it contradict the previous paragraph? Not necessarily. Please note, that such people were not actually 'conscripted' but were rather a part of the aforementioned knightly retinue. They were commoners who could have been selected on account on familiarity, talent for fighting, friendship or whatever other reason. Such a person, formally still a peasant, was equipped at the landowner's own expense and could have been also delegated to various martial tasks, such as helping to chase bandits or generally keep the peace. So, they were a sort of deputies, with a hint of bodyguards. The size of the retinue and quality of equipment would have depended on the wealth of the landowner and the peasant in question, but it was in the knight's best interest to arm his people well, as his life was quite literally depending on it.

There were places, where peasants were expected to fight, usually as a remnant of the early medieval common land defense mechanisms, such as the English fyrd (until 20th century it was assumed that there was one well-armed warrior per 5 hides or 600 acres in England what usually mean one warrior per village, although more modern research shows that the actual practice could have varied greatly; on a curious note, a similar system has been introduced in late 16th century in Polish-Lithuanian Commonwealth by the king Stefan [Istvan] Bathory). In early Middle Ages, especially in Central and Eastern Europe local rulers governing relatively small polities could have hand-picked men he armed at his expense, forming a so-called ducal retinue that was often a groundwork for the knightly class of professional warriors. Such people would have been rather well-to-do peasants (remember that even though the warriors were serving their master, their family could have still worked the land) who might have been granted larger plots of land in exchange of the military service for which they were obliged to provide weapons and armour at their expense. The Assize of Arms of Henry II or Henry III give us a good insight what weapons were expected from people holding a land of a specific value. Depending on the particular agreements, such people could have been drafted into units formed by the local nobles or tasked with defense of the border or patrolling the area. Thus, anyone not obliged to military service in a personal feudal contract with a lord had a minimal chances to see the battlefield.

In addition, a common sight on the late medieval battlefields, at least in certain places were the mercenaries who, having served in the army might have decided to become a full time soldiers. It was perfectly possible for a commoner not bound personally in servitude to leave home to join the soldiers. In would have been possible (although more common in modern period rather than Middle Ages) that the equipment would have been provided by the company itself with the wage being adequately docked until the debt have been paid (or, conversely, the wage for the people bringing their own equipment would be adequately higher).