+1 on the test drive.
We had the Volvo 740 wagon when my youngest was born (newborn daughter, 2 year old son, 6 year old son and 8 year old daughter), so we had the boosters in the rear facing third row that was accessible through the trunk and 2 car seats in the second row.
The vehicle was pre teather and the rear facing third row is sacrilege now although the children loved it, so times have changed.
I'd suggest the 4.5 and 6 year old children with the boosters in the third row and teach them to buckle their own belts (or have them help each other), the dash of the XC will tell you if the seats are buckled. Then the 2 infants in the outboard seats (I'd also suggest the removable cradle type of infant seats to avoid constantly fiddling with the seat belts or isofix straps) with the 8.5 year old in the center booster (you can keep it folded as a pass through for the third row occupants then fold it back for the 8.5 year old).
With some cooperation and some practice, I'm sure it'll go well but a test drive with the whole family would be ideal.
If you need more room, I think the Chrysler Pacifica scored well on crash tests and has many comparable convenience and safety features. Unfortunately it doesn't have an all wheel drive option at this time, which knocks it out of consideration for many including myself.
Best wishes to you and your family. Please let us know your progress:beer: