As much as all the museums above are wonderful… the OP oldest is 4 !!
Hamleys then lunch in the Rain Forest Cafe your kids will love it. Just make sure you and the Mrs eat before you go in there….. the food is shocking!!… oh and if you do decide to go, make sure you are there by 11:45 latest… they que out the door any later and is as much as the kids enjoy it, it’s not worth that kind of effort when the food is so poor for you.
Or take them to a show… Lion King?? Or is the Guffalo still on? Went the other week, it’s an xmas special but may still be on, same may be true for the Snowman… both are excellent.
Guess I’ve wondered into tourist traps for you but then it is hard not to if the kids are a propriety… but make the most of the fact that kids get up early… get out and hit Traf square and its HUGE lions and its big blue Cockerel, (my kids 5 and 3 enjoyed them) early before the masses
Have fun