How old are your dd's? it is quite exciting for kids, but like others have said the food isn't great and it is quite expensive.
We went to London yesterday and had a great time at the aqurium, it's really close to the London eye if you fancy that.
We went to the Medieval Banquet a couple of years ago and that was a really fun evening, again it is quite expensive, but the entertainment is good and it is a whole evening out. It is close to the Tower of London, you can get on a boat at Westminster and get off at the Tower of London. We left just as the disco started as that is more aimed at adults, but the rest of the evening if definately for both adults and children. The ebsite is www.medievalbanquet.com
I have heard that Bill Wyman's restaurant Sticky Fingers is great, we are going to try it soon, they have a kids menu and the prices seem reasonable, probably slightly cheaper than The Rainforest Cafe, the website is www.stickyfingers.co.uk
If you fancy a show, head to Leicester Square the day of the show and you can usually pick up tickets for up to half price, you obviously have to be a bit flexible about what you want to see though.
Enjoy your stay in London, there is so much to see and do I'm sure you will have a great time.