We have 4 children and drive an XC90. For holidays we keep one seat flat, making an l shaped boot. Next to whoever is then left in the third row we put a soft holdall, but it is lashed down securely. (There are lots of thoughtful little things like lashing rings etc).
We haven't needed a roofbox, even though we bought it when ds3 was under a year old and using a P&T as his main pushchair.
it drives like a car, which is an imporatant factor for me, unlike say a Voyager which is a bit wallowy and not very nippy on a motorway, (not to mention wider too i think). It's not that i want to burn lots of fuel and pollute the environment, but when overtaking a humungous lorry i want to pull away sharpish for safety.
We are going to change our car next year and I have looked at
another XC90
Grande Espace - looks good for a people carrier but actually not much if any more spacious than XC90 to my mind
Merc R Class - currently a six seater but new run will have 7 seats. However the middle row isn't very adjustable. Your central 3 passengers would have lots of leg room - nice but not needed if they're kids - but that row doesn't move forward much to capitalise on the large boot you'd get if you weren't always using the back row.
ML Merc - no longer made with 7 seats so not an option.
Merc GL - MASSIVE and v. expensive, so not an option.
X5 - ? Have test driven a diesel. Great to drive but would my boot be smaller than XC90 ? Not sure.