A friend's little one had a full cast earlier this year. He was about three. Couldn't walk at all at first, but was very determined and started pushing his little sister's walker around. Within a couple of weeks he was dragging the cast around like it was nothing. I think I even remember seeing him on FB jumping on the trampoline! So, as above, it was a very quiet beginning, lots of sedentary activities, but it didn't last long. I think his mum said sleeping was uncomfy for a few days, and it was hot, but it really didn't hold him back too much at all.