Kew Gardens is a great place to escape to if you're in London especially for the shade in the summer heat. It's 300 acres of botanical gardens but the nice thing about it is that there are no "keep off the grass signs" - you are encouraged to stroll around. Bikes are not allowed so it's a very safe place to bring kids. My kids love feeding the ducks and the carp in the ponds, looking for dogfish and catfish in the aquariums. There is an indoor playground called "climbers and creepers" and underground system of tunnels where they can "be a badger". We used to live 10 minutes walk away and it's always nice to get back there.