Kid's bikes are shorter and smaller and might have smaller wheels, but since your son is 11 I think you would be better off with a "real" 20" bike as it will have more room for him when he gets taller.
And as Brad said, any of the 20" onza bikes will be good to start off with.
If you think your son is actully going to commit to trials, get him one of the more expensive onzas as they will last longer as he gets better, but if you think he just wants to try it out i would say go for a cheaper one, as trials bikes usually have a pretty low second hand value.