There is a lot to interest children just in London itself if it's their first trip - children usually love their first sight of the Buckingham Palace guards, especially if you can be there in time to see them changing (about 10.45 and again around an hour later) and all the red buses, mounted policemen etc - so do just have a look around first.

For special attractions, you could start with a glance through this long list which has a bit of everything. Depending on how old your kids are and the sort of things they like, the Natural History Museum is really geared to children now - lots of things they can touch and engage with, not just look at. They would probably also enjoy a ride on the London Eye or a trip to one of the famous toy shops like Hamley's. And don't forget Madame Tussaud's or the London Dungeon - there really is more than you can do in a day. Have a great time.