good things to do with toddlers (under 3yrs) in Richmond/Kew?

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we'll be having a picnic at Isabella Plantation in Richmond Pk if the weather is good, but was wondering about a plan B especially if the weather isn't good. Any ideas?

Oh, and there'll be 3 couples with young children (3yrs-17months), but also a couple of couples without, so need to cater for childless adults too!! not too much to ask

Kew Gardens is lovely. There is an indoor area for young children.

THanks Squeaky - is that the climbers and creepers bit? I had a look on their website and couldn't work out if it was for older children (above 3yrs)??
But Kew is gorgeous, and if it is raining it'll still be lovely...