The British Postal Museum is not only a shrine for stamp collectors but a great place to bring the children. Especially exciting for children is the ride on the underground postal train that used to run underneath London along ‘secret’ tunnels, you can pretend you are a parcel! The museum is informative, interesting and fun.
https://www.postalmuseum.org 15 minutes each Kings Cross or Chancery Lane tubes or Farringdon tube and Thameslink