If you are looking for something fun to do this half-term, I highly recommend the Halloween offer that Fenton House in Hampstead is doing. Entry is their normal ticket prices, but you have to pay additional for the kids trails. It's not too crowded and it is a lot of fun for kids of any age. Like last year, there is going to be a story-teller that comes in and does a little bit for the younger children. If you live near Hampstead it is totatlly worth it.
www.nationaltrust.org.uk/events/2af6fde7-a6cb-47f6-b980-4407accb7c0d/pages/details