You could probably do with something with individual seats in the back rather than a bench type seat, gives more space between children. We have plenty of experience with trying to fit children in cars as we have 4 ourselves, and know it is not nice sitting between 2 car seats unless there is loads of space.
We had a Fiat Multipla for a few years and loved that, even had an extra seat in the front for friends, then dc4 came along and we didn't need to change the car (but the boot was rather small, something you often sacrifice for more inside car space.) Other than that, something like a Ford S-max, a VW Touran (a 5+2 seater) or a Citroen Picasso (or grand Picasso for more boot space).
Whatever you try out, please put 2 car seats in the back and have an adult sit in there while someone is driving around to see how much space there is. Also check for leg-room as being scrunched up is something that makes longer car journeys a nightmare, and remember to take into account your children are still growing, even if they fit ok now they might not in a couple of years when their legs are even longer (unless you have plans to change the car again soon.)
We currently have a Ford Galaxy and, as we haven't been out all together as a family for a few weeks, we put the rear seats down and have been using it as a 5-seater with a mahoosive boot - something to consider. (The rear seats fold flat and it's superb for putting luggage\pushchair\travel cot\moses basket\a million cuddly toys in when going on holiday.)