Thomas M.
Yelp
The Royal Botanical Gardens at Kew (better known just as Kew Gardens) are quite spectacular and offer a day worth of sight-seeing in good weather. On top of the open-air and greenhouse plants (mostly shrubbery and trees, flowers not so much), you'll get Kew Palace (qv), the Pagoda (which costs extra!), the Japanese gardens, a couple of galleries, the Royal Kitchen and so much more. Should be good fun for the whole family. Also a very nice park to just stroll through. You will be given a map of the area at the entrance.
Cheap it ain't (a single adult ticket goes for about €20 , discounts for families and seniors are available), but seems reasonable enough. Best way to get to Kew is to take the Underground (District Line to Richmond) and alight at Kew. It's a suburb and really nice, quite removed from the bustle of central London. From there it's a 10-12 minute walk to the gate. ALWAYS book your tickets online, makes it so much easier.
My advice would be two visit the large greenhouses first, the rest later. It's all attractively designed, and there are activities for the small fry, too. If you need a rest, have a seat at their cafeteria-like restaurant The Orangery (not cheap, but decent food). A normal visit for a single visitor should take 3-4 hours at least. With children, give it a full day.
Recommended.