you local bike shop may be able to get these, which might be close enough: http://www.jbi.bike/web/checking_pro...t_number=63238 24x3 tubes seem to be pretty common, so you might have to make due with those.

irony! you are discovering why these bikes are sold so cheap at walmart- they are disposable junk. once you get a flat, the bike is just about useless. put a kid who is going to ride through grass and into curbs on those cheap tires, and you have a constantly un-rideable bike.