i dont see how a child who believes their presents come from a stranger is more innocent than one who thinks they come with love and hard work from their devoted adoring parents.
fwiw - ive been using the old 'FC wont bring you any presents if you dont behave' line on mine too, so im no innocent in the pulling the wool over their eyes stakes either, but under close cross examination i dont think i would swear blind its true... i dunno, the oldest of mine is only 4, dss is 7 tho - if he asks? will prob make unconvincing 'yes its true' noises as its not my place to disillusion another womans child.
i just dont get the hysteria about it all tbh. and im not saying anyone in particular is hysterical about it... just a general feeling.
like you say, they do see things like, the presents under the tree having the same paper on that was in the supermarket shopping bags, and returning stuff to the shop when it doesnt fit, and empty boxes in parents bedroom and they even quite often find the present hiding place...
its a nice enough fantasy to all go along with and collectively, as a family, pretend is true, i guess its part of the magic, but really, do they need to utterly believe it? is there any harm in everyone knowing deep down that its just a fantasy? can they not just pretend to believe? ime most kids do anyway. then again, maybe i grew up in a particularly un-innocent childhood? nah, dont buy it, i was naive as F as a kid. (and I knew he wasnt real)