If you are planning a trip to London with children, Kew Gardens is definitely a must. We are pretty bummed that this was our first visit to this wonderful place! We surely would have come often during our other stints here, had we known.
This is one of the few sites in London that you have to pay admission fees, but it's worth it. The glass houses are beautiful, the grounds are breathtaking, there's an awesome tree walk that's really fun (though it does sway in the wind and you can see through the floor to the ground, which is a bit trippy), an amazing playground/discovery center for kids, and you don't have to refrain from walking on the lawn... in fact, they encourage it! We went on a Saturday and thought it would be packed, but at one point we went at least an hour without seeing other visitors. We felt like we had the place to ourselves.
It was so picturesque.