Went to the park yesterday after feeding the ducks behind the library (slept too late and weren't ready to go to the zoo until 9:30. Would have put us there with all the other visitors and we like to go early and have the zoo to ourselves).
After watching the kids play for a while, I noticed these:
Only that's not the way it is. You have to swap middle feet and little feet, because the 2 year old wears a size larger than the 4 year old! He'll be catching up with the big feet pretty soon.