If you are speaking of pre-industrial Britain and Europe, there's a rather fuzzy answer. You would expect a great number of people to be living by farming, and there would be children involved in that, whether it's going out and bringing in the cows at dusk, shelling peas, or tagging along with their mothers helping in gleaning a harvested field. But we could think of this as doing chores: tasks, but not day-long constant hard labor. The simple reason was that children aren't big and strong enough to do day-long hard labor yet.

It would be when they were around the age of 13 or 14, when they were getting bigger and stronger, that you would find them apprenticed or indentured, or sent off. Apprenticeships were somewhat straightforward- the boy or girl moved into the family of a craftsman or tradesman and learned the trade in exchange for their labor. That could be anything from blacksmithing to being a cloth merchant, and sometimes if it was lucrative- like, say, a merchant in foreign trade or a watchmaker- the family would have to pay significant sum along with bonding their child over. That bond meant that, for the term of the contract, the child was not free: if the blacksmith's apprentice ran off, the blacksmith could legally force him to return. There were often good reasons to do so, as in the first years the apprentice would be untrained manual labor, maybe costing more in food and lodging than he earned: but after some years of training, he was free skilled manual labor and the master of the shop would get back all he'd invested in the training. Advertisements for fugitive apprentices in the 18th c. are not uncommon in newspapers, and typically offered rewards.

Girls and boys could also be sent to serve as servants in the houses of the noble and wealthy. But there would also be a practice of even ordinary farm families trading children- sending their daughter to another farm, taking in another farm's daughter. Parents today often notice that their children are much better behaved when visiting at a friend's house, and it was a stated reason then that children were sent to other families not only to learn, but to teach them to mind. And to learn to serve, as the pre-industrial world was one filled with hierarchies and service to someone higher was typical.

There would be always a significant number of children who'd lost their parents. For Britain and the British colonies, at least, there would be a local magistrate ( in England, working with local clergy who'd be doing much of the alms for the poor) who'd indenture or apprentice the orphans to local families or tradesmen, or even pack them off to the colonies as indentured servants. In these cases, the children were sometimes younger than 13: how much work they were expected to do, how they were treated, almost certainly varied- but without parents advocating for their benefit, you suspect they could get worked very hard.

Laslett, Peter.( 2004) The World We Have Lost: Further Explored 4th edition, Routledge.