Sorry. I keep adding to this. I do this a lot.
London Zoo is great ( even though it’s a Zoo) for a morning, short walk to Camden Market along the canal after, street food lunch. Or do that in reverse starting off with a canal boat trip from Little Venice, to Camden.
Hamleys too, then walk down to Piccadilly Circus, through to Trafalger Square, Downing St and Westminster, ending at Big Ben and the river.
Natural History Museum is great.
Hard Rock Cafe at Piccadilly is expensive, and you need to book, but a treat for kids.
Hop on and off tourist bus, is good for orientation, if you’re not that familiar with town.
Go see a show. Loads to choose from. The Goes Wrong people have shows good for kids. Depends on their age.
It will be expensive, but don’t keep count it will lessen the experience.