Having taken kids XC skiing in sleds, here are a couple of things to think about
-they will generate very little of their own heat, so insulation is foremost. Think something like a sleeping bag as you do not want to constrict them.
-Extremities ( hands) are really prone to getting too cold as they like to pull them out of whatever you've bundled them it. This is why a sleeping bag type thing is good, you can close it up around the neck so the hands stay IN and WARM.

have fun