He will be 10 at the time of the trip, yes.
Where does a 9 year old learn about Hampton Court? Impressive.
Research the Premier Inn website for accomodation.
There are wonderful parks with playgrounds to enjoy like the Princess Diana play ground which reopens in 2026. https://www.royalparks.org.uk/visit/parks/kensington-gardens/diana-memorial-playground
Would he want to watch the feeding of the pelicans in St James Park, walk in Richmond park and see the Fallow deer or enjoy the green parrots near the Peter Pan statute in Kensington Gardens? Watch motorized mini sail boats in the Round Pond near Kensington Palace.
As you’ll be in Westminster and I’m going to assume walking by Buckingham Palace make note if the Royal Standard is flying it means the King is in residence. If the Union Jack is flying it means he is not there.
Any interest in farm animals? Could visit Mudchute Farm. Does he like horses? If yes, then watching the Horseguards mount up to proceed to the palace for the Changing of the Guard is fun. That way you can avoid the masses who want to witness the ceremony.
If not afraid of heights the London Eye or the Tower Bridge Experience. Speaking of Tower Bride, upload the lift schedule.
The Imperial War Museum is interesting. Museum of Science as well.
Lastly, does he like dinosaurs? An outting to see Crystal Palace Dinosaurs might be fun. https://cpdinosaurs.org/
Unless he’s an urban kid and is use to a metro riding the London Underground will be something new. I’ve found boarding after 10am and before helps to avoid commuter travel. Also if train is crowded wait for next one. I always get a seat in the second to last car.
There are numerous food halls &
street markets. If his palate is agreeable try the Nandos, Bill’s, Dishoom, and Pret a Manger chains. Countless pizza restaurants as well.