The one I got from RoadID came in size Small and Large I think. Anyway, it fits my wrist which is about 5.75 inches around. But I can see how someone with an even smaller wrist, including children, could find it too big.
This Canadian company has an interesting product:
http://www.vitalid.ca/store/scripts/default.asp
I don't think I'd go for the helmet tags but they also make bracelets and shoe ids.