disabled mum here - a Londoner so very well versed in getting around step free
You can change lines on the tube!!!!! you are not restricted to only northern line. That is just your starting point!!!!
also London buses are all wheelchair / buggy accessible - not recommended at peak time when busy but can be fine off peak.
use tFL planner with step free station selected - particularly as you can access Tottenham Court Road from northern line. Hop on the Elizabeth line there - great trains - modern and spacious and lots of accessible stations. Could go to Farringdon to the post office museum - really interesting
or get to waterloo - once you get through the maze of interchanges and navigate the lift system (either end of the station to get to the ground level) you are in the south bank - London eye,aquarium, shrek exhibition, Paddington experience, London dungeon - cafes, street performers, south bank centre, wander down to tate modern, take a river boat - absolutely loads going on!!! Wss as ok ti Westminster station from there - loads of lifts - on the district line snd off you go to tower bridge / see the Tower of London etc…
… being step free doesn’t stop you - just have to be a bit more creative in your routes and may involve some walks above ground or to find the lift tucked away in corner.
London is full of awesome and many free things to do with kids - yes King’s Cross is great if you want to people watch from the gallery with a cupa or to look at the Harry Potter shop or go to the canal museum or British library etc - but so many exciting places to go beyond King’s Cross and the northern line with a bit of planning!
I look at where I want to go first - then work out how I can achieve it. My routes due to my disability are not always the most direct or quickest - but I get about to where I want to go.
enjoy !