Man, been there, done that, though only with two kids. The day hotel is a decent idea, but my goal on red-eyes is to have kids that are exhausted enough to sleep nearly the whole time. You could consider resting in the morning briefly at the day hotel, but I would hit the town next. I think LegoLand is a good idea, too. Alternately, you could just take the train into town. If the weather is nice, you could walk around the parks, get some lunch, etc. If the weather is bad, the kids would love the Science/Natural History museum complex, and the V&A Museum of Childhood is also very good.