LL is fantastic, a day is enough if you plan to really go for it and stay to the end (remember, if you're in a queue at park "closing time" they still let you ride, you can often bag a short queue for a major ride if you time it right. Also note the food outlets close early, so do early lunch at the pizza place and then take a picnic tea).
HP is great, just over 3 hours was enough for my DD at age 9, but we could have spun it out to 4. I found it enough just for a day trip as getting there is quite a long journey, so whatever else you plan that day keep it low-key; maybe HP in the morning and then travel back to your hotel for a rest, then Hamleys just before closing time and dinner nearby?
In London, definitely skip the Aquarium unless you're a Merlin pass holder and can get in for free. Shrek is fun, don't bother with London Eye as it is slow and frankly gets dull after about 10 minutes plus terrible queues in peak times. Instead go to the great little playground/garden on the southbank as you walk from Waterloo up to Shrek; the riverside views on that walk are lovely and there's usually a few buskers. There is also a regular street food market near Waterloo station which could be useful to know.
My dd absolutely loves the science museum and Wonderlab, Natural History museum also a massive hit. In fact she enjoyed the British Museum too, although unless you have a special interest eg Egyptian mummies, it may not be top of your list.
Princess Diana Memorial gardens, big tick if you're nearby.
In summer hols usually August there's the Underbelly Festival, this year in Cavendish Sq. and Earl's court. In the evenings it is adult but they often do fab, quite cheap kids entertainment/shows in the daytime and sell street food - no events announced yet but worth looking out for.
London is exhausting and you will need to pace it a bit to avoid the kids getting the grump. I'd suggest try to just pick one big bookable thing per day and build a day around that. My dd loved going on buses and a river boat.
My dd did enjoy the Tower of London but I think it would have meant more age 10 or 11 and it is expensive and I wish I'd just taken her someplace else instead that day.