We went to the observatory in December and it got pretty crowded as the day went on. The 5 year old did enjoy the planetarium shows.
I think we'll have to play it by ear really. It's the last couple of days where we're meant to avoid crowds that we're there. If we're in museums that are free to go in then we can always go early and leave if it starts to get crowded.
To be fair, she's meant to avoid people with coughs and colds too. In our household as it currently is we'd have to leave her home alone for 2 weeks to manage that.
Museum of Docklands might be good. We're staying out that side of London anyway.