Just come back from the Science Museum with two exhausted kids - was definitely the right thing to do on a rainy afternoon in London. My girls are 2 and 5 and there was loads for them to do even at this age - they both particularly enjoyed the 'In the Garden' interactive play in the basement area and it was tough to tear them away from it when we had to go. We also bought tickets for the IMAX Deep Blue Ocean which was really worth it (although some bits were scary for the younger one). The bookshop on the way out has some great books, so even though the museum was free, I came home having spent quite a bit (but happy with the purchases!). Food, toilets and baby changing facilities were all excellent.