The botanical gardens are a surprise hit for kids. The wide open spaces are an obvious boon for children squeezed into London homes but when you add in the children's play facilities, gorgeous cafes and the Xstrata tree-top walk its a great way to spend a whole day.
At Christmas they have an outdoor ice rink. You 'll feel like you've fallen into a Victorian Christmas scene as your family skate in the shadown of the palm houses but make sure to book - it fills up weeks in advance.
http://www.kew.org/