The first year we went with all three of our kids we got a free upgrade - we had already paid to upgrade to a large saloon but they were out so we got a free upgrade to a 7 seater mpv. Kids absolutely loved it and we have always booked a 7 seater since. There is not a massive price difference between a large saloon and a 7 seater, I have just paid £590 for a 7 seater for three weeks in Aug.
What you have to remember is that the US 7 seaters are nothing like the uk ones which frankly I think are a waste of time. I looked at 7 seaters and the ones with the best room were the Kia Sedona and Grande Voyager. In the end I decided that I wasn't buying a 7 seater for the odd occasion when I have another child in the car. However, we do always hire one in the US for the fun factor for the kids.