London ideas please

[WestYorkshireGirl]

Off to London for an overnight visit with 4YO. Going to to NHM and Science museum, but not sure what else to do as she won't walk all day. Staying in Kings Cross. Thought about a London bus tour but they seem very expensive. Don't want to go far off zone 1 and main tourist places due to time and not got a huge budget. Any ideas appreciated! TIA

Someone with a better knowledge of bus routes than me will no doubt be along soon to recommend the bus to get to take in lots of sights from the top deck without paying bus tour prices :-). Kings X not far from regents park with ducks and playgrounds if it's a nice day (zoo too of course).