Definately Dulwich Park, hire them a bike, keeps them occupied for an hour (and wears them out!!). Further afield Christmas Tree Farm nr Downe, Bromley way is good for an afternoon. Cheap and cheerful, the kids love feeding the snimals. Also on a wet day going up to the museums in London is good, especially the Science museum, lots of hands on things to do. Train from East Dulwich station (no steps, soooo much better than slogging the buggy up all those at Peckham Rye), train to London Bridge, which has lifts to the tube. Only steps then are at South Ken station. Museum of childhood in Bethnal Green is good too, but lots of stairs and no lifts on the tube line!