My youngest started MTB racing when he was 6 - not the kids races, the regular 10K in the WV MTB race circuit... so yeah, if the kid is generally in good shape, a rail trail should be no problem. BUT - take snacks & some G2 just in case... circus peanuts go over well with that age, and they have the same stuff that Goo has in it.
__________________
As with mud, life, too, slides by.