I've spent a lot of time in England. You already know the tourist attractions, so I'll only comment re: your little kids.
The usual wonderful tourist sights won't be good for little kids so young as yours.
For them, if you go into London the Hyde Park amusements (swings, pedal boats, chase ducks around on the grass) would be fun. Take warm jackets.
Also maybe fun for little kids: riding atop 2-deck buses, tourist boat ride on the Thames, go out to Greenwich Royal Observatory & climb around the old wooden sailing ship, London Eye ferris wheel ride (maybe too scary for your kids).
Also, some of the museum-type places do a tourist thing called 'rubbings' where you pay to put paper over a gravestone and rub its design onto the paper. Your kids might like that. I think they still have that inside Westminster Abby, off to the side, & at other places too.
You can hop a train down to Brighton where your kids can see the aquarium show there and also the Royal Pavilion (for your interest, boring to kids).
They might be old enough to enjoy the wax museums in London.
Be extremely careful with your kids at rail stations because there's usually no safety guardrail between the landing where you wait and the tracks. At some places express trains barrel past the landing at high speed with an air blast, a serious danger to children.
There's a big animal park far west of London, I think southwest of Windsor. Kids would probably like that.
With little kids on a trip its best to keep on the lookout for parks to play in. Especially parks with other kids around their age to play with for a little while. Been there done that.
Also,getting little kids young as yours to eat different things than they're used to can be a challenge. Ice cream!