Woohoo a topic I have experience of.
I have a Cosatto Hubbub I have used behind my (driver seat) with a little comprimise on space for my 2.5 year old. This is ISO-FIX and fits from 9 months on, used a maxi cosi pebble when she was younger behind driver seat with no real issue when using the seat belt fitting rather than the ISO-FIX fitting.
Now eldest is bigger 6 she has a Recaro sport hero which is seat belt fitted rather than ISO-FIX. her legs are just about long enough now to tuck under passenger seat and allow the wife room. Although on a family trip this year I sat her up front and the wife in the back.
Worst time was probably when they are around 4 and are a bit young to put up front and legs almost have to stick out horizontal from the seat bolster. It was tight up front then
