The Fitbit Ace LTE is Google’s kids watch, although it wasn’t yet available when OP first bought watches.
With Google moving their smartwatch efforts to the pixel brand I'm worried about longevity here. It also has a lot of the downsides of the Gizmo: 20 contacts max, texting only through the app, can't interact with friends. If I'm going to get new ones for the kids I'll probably do a TickTalk or Cosmo that don't have these drawbacks.
I almost started talking about how I think the best solution is really just a customized/locked down normal smartphone but I’m not sure how helpful that is to the discussion.