A delicate (relatively speaking) electronics device lasting more than a couple of years for someone who's currently ten? You MUST have raised one gentle child there.
Personally I'd get the 8GB and forgo any idealized notions about it lasting into his high school years. Besides, even IF it physically survives that long, Apple will have dolled up any future devices so much that the world (and probably your child) will be longing for an upgrade much sooner than later.
I'd say don't worry about the distant future. Besides, filling up an 8GB unit with music, apps, videos, games, etc. could easily have enough financial burden of its own to mitigate choking full the player's flash memory anytime soon (assuming that you'll want your child to do it the legal way).
we are getting an itouch for my 10 year old, but there's quite a jump in gigs (and in price for us) between the 8 and 32 gig models. seems like the 8 gig would be the way to go for a 10 year old, but i know he'll have it for the next few years, so will it be able to grow with him as his music and app tastes develop. thanks.