Seems to me that my kids heads were big enough to wear XS size adult helmets, but I am thinking from over a dozen years ago. I’s ask in a general forum about it, and see what the W2W crowd is doing for the kids. Not sure that kart racing org’s require Snell.
Bill, there is another Snell category labeled as K2010 that works for karting. Again, Bell makes a few models in XXS for youth karters.
My guess is that the kart standards won’t update as frequently but they may eventually get into a groove once they see technologies trickling down from the highest levels.