The one we always stayed in is closed for refurb so i suggest you look at the travel website sleeps5.com or roomfor5.co.uk then check the hotel out on trip advisor. my best advice for london trips with kids is to either go really cheap or really expensive/top end on the room. if you go in the middle you are nearly always disappointed with the quality etc. good luck
You can get rooms with 2 Queen size beds plus an additional pull out bed, the rate includes breakfast for all. When we stayed there they were doing a deal where if you had your evening meal there children eat for free.
It is just over the road from the Edgeware road tube station, and a 10 minute walk from Marble Arch.