Suggestions for family friendly London places to visit

We are planning to meet some Australian-based-UK-visiting friends in London on a quick daytrip.  Their son is 1-year-old, our son is 10-months-old. We will arrive at Liverpool Street but are happy to Tube to somewhere else within reason.

All you London based people-any suggestions for places to go/meet up? I've been to London plenty of times just never with a baby!

If it matters, it'll be a weekday.
Thanks!

If it's a dry day - Princess Diana Playground, Kensington Gardens

http://www.royalparks.org.uk/parks/kensington_gardens/diana_playground.cfm

It's free and there's a cafe next to it.  There's a special area for the tiniest children.  Bring a stroller and it's lovely to walk around Kensington Gardens/Hyde Park and see the Albert Memorial, Peter Pan sculpture, serpentine, Diana Fountain, etc.