I'd try something like a budwing as Nick said. Anything in the genus Sphodromantis, Hierodula, or Rhombodera would be neat. For small children, much like toys larger is better.
You could always have the kids look for food to catch, you could identify it, then feed it to the mantid. These mantids could tackle anything you can find outside so it would be good in that regard. Getting mantids that are picky might be a bit cumbersome for young children.
Also, if you get some US species it would be good to follow the life cycle. For example, at the end of summer put the adults outside and next summer see if you can find any oothecae. You could always keep a pair to make sure they mate and reproduce.