Science museum has lockers & a picnic room in the basement with an interactiveroom in the basement for toddllers.
Fir the natural history museum, use the entrance next to the science museum to avoid the queues at main entrance.
british museum has a lunch / locker room in the basement of the great hall. Cheaper using the lockers for a £1 than £5 service charge in the cloakroom.
The Brunswick centre which is a 10 min walk from Brit museum across russell sq park has restaurants & waitrose. It is across road from russell sq tube station on the right.
Budget meals avoid museum cafes as they are ridiculously expensive. Buy lunch/ drinks h from the tesco/ sainsbury express shops and eat in the picnic rooms provided. Even cake is £2/£3 per slice for that you can buy a whole cake and slice it yourself. I just,buy tea from the cafe as I bring everything else.
For evening meals, head to covent garden early (6pm) and take advantage of restaurants offering pretheatre 2 course dinners between £7/£10 per head. If you go eat somewhere like giraffe try & pre book as there is often a half hour waiting time.
Jump on any bus heading to oxford st/ trafalgar sq (no 24 from victoria station is good as it goes past parliament, trafalgar sq) to see all the tourist spots cheaply from the top deck of the bus without spending a fortune on tourist bus tours.
On the escalators in the underground, walk down on the left and stand on the right.