The Royal Horticultural Society garden at Wisley is very kid-friendly, with a spacious glasshouse, an interactive 'Root Zone' worthy of a commerical science centre, and a teaching garden that offers "hand's on experience to gardeners of all abilities and ages."
The garden was designed by Cleve West and features an "environmentally sustainable garden shed" with a sedum roof. (I think it's the sedum roof that makes it environmentally sustainable.) I visited the garden on a chilly and raw December day, but the garden was looking quite pretty and the sedums, of course, were holding their own.
