Help me decide where to take my children (London)

[Wallabyone]

We live just outside of London and our half term is next week. I'd like to take my eldest two in for a day out, but am being really indecisive and would like some help.

We've thought about the Royal London Observatory (too far?)
Possibly the Postal Museum, or the Museum of London? I've not been to either of these.
Not the Science Museum or the Natural History Museum, nor the Transport Museum.

Any ideas? They're 7 and 5.
Thank you in advance!

Thanks so much to you all for your very helpful suggestions. We had a brilliant day at the Tower of London yesterday, and I've banked your other ideas for weekends/other school holidays 😊