You presented what may be a bit of a trick question. Roof Top Tents aren't really designed for family camping. Yes, there are some huge ones where you can and I know the companies out there will happily sell you one but I wouldn't do it. Here is my reasoning, a well designed RTT is simple, fast, light and convenient. The big ones are not, I don't even think they are safe, way too heavy for most family runabouts.
I would suggest looking at a RTT the same way you look at a sleeping bag. Get what you need for your journey and nothing more. A large sleeping bag isn't better, just bigger and may be worse because it is heavier. Same idea applies to RTTs. Get one that is as simple as possible, it is easier to put on your vehicle and you will use it more. When family camping, go old school, ground tents are great, even if you use a RTT you need a place marker for your campsite. If you bring a RTT kids will be up there, it is like a tree fort.
Since I'm on something of a rant, please excuse me, but I see a lot of marketing hype out there, even on these pages. The reality is that the inexpensive folding tents aren't really set up to be on the vehicle full time, fuel mileage, durability ...., so you want a way to get them off easily. Anything much over 100 pounds is difficult to load, the noise at the other end gets loud (from the wife). So, you want to be able to lighten the load, have a mattress that is easy to take out, a good one is heavy. Get the lift weight down to less than 100 pounds.
The other thing I've noticed about RTT is they are vehicle dependent. Family camping is more about parks and campgrounds. You can set up a ground tent in the campsite, while the RTT is where you parked it. So, while a RTT is a good thing, it doesn't necessarily replace a ground tent. I use both, my next trip in two weeks, the RTT is staying at home, we are going to be in a lot of parks and a ground tent makes more sense.