While looking for a museum to take my childrens for a last bit of outing before the school start, I decided to check on London Transport Museum. Just discover that you have to pay to get into the museum. Since when?
Quite expensive, too. I wouldn't bother unless you have a specific interest in transport or memorabilia. There's not much scope for kids to crawl around buses etc.
How about the Museum of London? Free and fascinating.