I gave them to my kiddo at the halfway point of a ski outing. He was in the pulk and he held them in his mitten. It both kept his hands toasty and he was occupied with squishing it as well. He didn’t complain about it getting too hot. I also used it on another trip to keep my camera battery from getting cold sapped. I was able to take pictures the whole trip and it was -5 when we set out on our journey.
I think the larger back size could be placed over a child in the pulk or at the bottom of a chariot to provide some heat to a child’s feet.