There are a lot of rooftop tent solutions for larger families but not a whole lot here in the states. Here is a good example.
Two medium OverCamp tent side by side. Oh, doesn't look so big on the little army trailer, no this isn't an army trailer. This is a custom Hummer trailer, extra large. You can walk under these tents.
This is an expedition rig we made up years ago, had side tents and all the goodies, cost no object. Medium OverCamps I believe are special order for North America, they are close to six feet wide when open, so you are looking at something like 12 foot wide of tent in this picture. One home for the kids and one for you and a couple more kids. The OverCamp is one of my personal favorites, used a small one last year for a month long exploration of the Copper Canyon.
Sounds perfect, but not so fast. Here are the drawbacks with the large tents. First, the OverCamp, as bright, cheerful and playful as they look, are expedition grade equipment. What works well for expedition, may not be what you may want for a family. For example, the whole side of the tent rolls up, good for exploration in the tropics but it also becomes a great jungle gym for the kids. The other is weight, again the tent is very strong and the literature says something like 130 pounds but I think it is more like 140 each for the medium size when built for North America, they don't come in large because of this. Two would be 280 pounds on the roof using load bars rated for 165 pounds. A potential problem, and on most vehicles, you can feel anything over about 145 pounds on the roof. This is why the picture shows a custom trailer and it was engineered by the trailer manufacturer.
There are times where I admit, a trailer is needed. I'm not a big fan of trailers, I've destroyed at least three, maybe four, no make that five. Think about getting two rooftents, different styles. Perhaps one hard top rooftent for going long and then having another tent on the trailers for the kids.
Rich H