Ok, lets break this down a bit

I've read that it's believed today that trench warfare and many of the battles of WWI were basically pointless slaughter that didn't decide anything

Its certainly the case that this is the belief of many people today, but this doesnt mean its the unanimous opinion and it also doesnt mean its true. There is a substantial body of work that disagrees with this point of view, including historians like John Terraine, Richard Holmes, Brian Bond, and Gary Sheffield

Indeed, from the British point of view its somewhat baffling that a war against a millitarist Germany that has invaded Poland is a good and just thing, but that a war against a millitarist Germany that has invaded Belgium is pointless and futile.

Its also unlikely that the Polish see the war as futile and pointless as it brought an independent Poland into existance.

Other countries like Serbia and Belgium were fighting for their continued national independence against foreign aggresors. Both Germany and Russia stood much to gain from a victory while France stood to loose much in a defeat, and Britain would find itself with "the enemy on the counter-scarp" if Germany won.

Its hard to see how any of this is pointless or didnt decide anything......

Did the public realize this at the time?

As Gary Sheffield outlines in Forgotten Victory this point of view was not widely held at the time and in fact came into the mainstream in the 1960s and in particular after Alan Clark wrote The Donkeys, named after the supposed repartee

Ludendorff: The English soldiers fight like lions.

Hoffmann: True. But don't we know that they are lions led by donkeys

which has always defied reliable attribution and is actually quite odd when, as John Terraine points out, you consider that Hoffmann was an eastern Front soldier and never saw a British soldier or officer in action.

Did people ever start to believe that it would be sensible to avoid service on this basis or try to protect their children from service? Was there ever a movement of draft dodgers or deserters in Britain or anywhere else?

In the UK, conscription was only introduced in early 1916 - up to that point all the soldiers were volunteers. While there was undoubtably some of this behaviour, there is no record of it being widespread either in the UK or anywhere else.

It is worth noting however that there was fairly widespread mutiny in the French army after the Nivelle Offensives and that German society by 1918, was experiencing sporadic outbreaks of revolutionary activity, most seriously in its navy.