The St Martin-in-the-Fields church holds numerous commemorative brass plaques and you can go down to its crypt to make your own. You can pick any you like from a large collection of replicas of more than a hundred brasses from all over the UK.
It is surely a great place for families with kids who will definitely enjoy the creative activity. Don't worry about papers and crayons, you'll get them here.
The staff will show you various techniques of "rubbing" and since it is said to be a very calming experience, it is well worth the visit. Plus, you get to keep a nice souvenir!