If you're worried about the kids staying up and complaining what about doing something totally catering to them that day? A theme park, aquarium, something really fun that will keep them awake and help de-jetlag them quick!
I know that that always worked for me as a child, and now with my own three year old I try to do something that I KNOW she'll want to stay awake for after a redeye flight. Being dragged around london on foot with my parents as a pre-teen I wanted nothing to do with it and was falling asleep wherever I could sitdown.
London has FANTASTIC childrens options for all ages. We'll be doing this same thing next month.