Try the Ramada at South Ruislip, Next door to the tube and reasonably priced, or try Hotels in the Kings Cross Area, just come back from a few days there but prices are variable.
Warning, there are huge numbers of school parties at the museums mostly well behaved especially if from overseas but........
London Eye has a security check for bags etc., which catches people out just be aware, it can slow things down a tad.
Otherwise have a great time.

You've probably already got it in mind but I can't recommend the Science Museum highly enough! For both adults and kids!

If you want to stay closer to town then the hotels around Russell Square are usually pretty reasonable and its a handy location. The President, The Imperial and the Royal National are all decent value.

There's also the Ibis next to Euston station.

Wrong time of year for the OP but if anyone is ever looking for no frills cheap bed and breakfast in London during July, August and September, have a look at Imperial College London's halls of residence. We stayed there for a gig in Hyde Park and the location is second to none for London's most glamorous quarters.

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Always stayed at the St Giles hotel, Not too pricey for its location.

Hamleys toy store, good way for the kids to spend an hour or two.
The wallet might take a hit though!

Way to late now but personally Borehamwood or Watford then get the train in.

How did the trip go?

My little girl said she did not want to go to loads of museums etc and she wanted to go somewhere for fun.

We ended up in Blackpool for 4 days !!!

Looks like it will be the summer holidays when i drag them down there now.

Harry Potter tours?

OK, from someone who lived there for 15 years and ended up showing someone around for the last 13 months...

Grab an Oyster card, pay the deposit and get a refund when you leave, they save money lots of it.

Kensington Gardens - Squirrel fun, very tame, they will feed from your hand and even demand food. Take some monkey nuts with you. Good for relaxing with a cuppa while the kids can have a bit of fun.

The boat from Camden Lock to London Zoo and then the zoo, a full day so wear some comfy shoes and take a packed lunch or supermarket sandwiches as food is quite pricey.

Open top bus tour, seriously the best value you'll get in London.

Riverboat trips down to Greenwich and get the DLR back.

Depending on what they are into you have all the museums, the war museum is always good for a special exhibition.

The M&M store in Leicester Square, just leave the wallet at home

Check for stuff like changing of the guard or any other royal comings and goings.

Half price ticket booth in Leicester Square for a cheap visit to the theatre.

Keep your eyes peeled for London Transport offers.

Walthamstow market, used to be the best general market in London.

You don't even need an Oyster card now if you have a contactless bank card, You just use your debit card and the Oyster fares / caps automatically apply.

Just had a trip to London with 2 daughters aged 5 and 7. We stayed at premier inn in Belsize Park. Not too far off M1 and 10-15 minute tube journey to central London. Stuff we did:
- quick look at Buckingham Palace, Westminster, St james Park etc
- Natural History museum (get there early, queues soon start growing)
- Tower of London (£60 for a family ticket)
- The Railway Children at Kings Cross station, with a real steam train (although not actually under steam)
- National Gallery
- Rainforest cafe at Piccadilly for eats.
- potter along south bank, walk over hungerford or waterloo bridge for great views along the Thames
- climb the Monument (£4 adults, £2 chilrden - 311 steps)
- Explore hampstead heath, good views of central London from Parliament hill

Having lived in London for 6 years in the late 90s/early 00s it was a different experience going back as a tourist. The London skyline has also changed during that time, with the Shard and other skyscrapers being built. The use of contactless cards on the tube is a great idea.

Oh cool, didn't know that.