If the children love dinosaurs, The Natural History Museum's your place, my kids made me spend hours there when they were young, I'd be begging to sit down, long before they'd had enough! I wouldn't be too over ambitious in trying to squash loads in given they're still quite young children can get overload fatigue, as indeed adults can too.
I'm inclined to agree with other suggestions here, foot slogging can be hard on children, the tube can be unpleasantly crowded at rush hour times. I would look into a double decker tourist bus trips or a down the river excursion where maybe the main sights can be viewed easily. What London has in abundance are parks and green spaces where the children might like to run around in to let off some steam,the more central ones are Hyde Park, St James's Park, Green Park, Regents Park, or you could head off to Parliament Hill on Hampstead Heath which has a great views over London, good for picnicking if you don't want to spend a fortune in restaurants. I think there's a lido up there somewhere too. Also there are swimming ponds on Hampstead Heath which are very popular.
Hope you have a nice break however you decide to spend your time there.