Aww, those kids adore their daddy! You can tell they are used to roughhousing with him and he's a hands-on Dad. I imagine when he comes home from a day of Prince-ing that Catherine greets him with,"Thank God, you're home! Can you entertain them while I make dinner?"
Charles looks a little feeble in that picture but still one of the cutest pictures I've ever seen of him showing some personality.
Happy Father's Day to them both! Although, I thought it was an American holiday? And happy birthday, William!