Shearwater,
I don't wish to rain on anyone's parade, but minivans are generally NOT good tow vehicles. I learned that 17+ years when we were looking to get into RV'ing and trying to find something to tow with our Odyssey. It quickly became apparent the Odyssey wouldn't tow what we needed for a family of 6. The family you're talking about is a family of 5, so perhaps a bit less of a problem.
Most minivan's have a published tow capacity of about 3,500 lbs. However, that is with a single driver of average weight (150 lbs. or so). Add people into the van and all of that comes off the tow capacity. Add cargo and that comes off the tow capacity. When you connect the trailer to the minivan, the tongue weight is added to the minivan. The minivan's are front wheel drive, so putting the weight of a trailer on the back unloads the weight from the front of the minivan, lessening the traction at the front wheels.
I'm not saying it can't be done, but you have to look at the weights very carefully. Seeing as they're already tent campers, they might be able to work with a minimalist solution. We knew we were going to be "sissy campers" and wanted more conveniences than anything could offer us that was safely towable by our Honda. As an engineer, I tend to read specs and limits and take them to heart and I didn't feel comfortable carrying my family in a vehicle which was operating near, or over, some of the specified limits of the vehicle. It might work fine 99% of the time, but during an evasive maneuver, it might make the difference between avoiding an accident and causing an accident.
If they do go through with this idea, they should also investigate to see if they can add an auxiliary transmission cooler for their minivan. Honda made one for our Odyssey as part of their "tow package". I think that's a prudent step for any vehicle which will be towing.
Don't get me wrong, minivans are GREAT. But they are NOT great at towing anything of substance.
Oh, by the way, I HOPE they don't go as overboard as we did. We started out looking for something we could tow with our Odyssey and ended up with our Class A which tows our Odyssey behind. We've been thrilled with THAT solution for all of these years, but it did cause us to adjust our budget upward just a little bit!!! LOL :)
Good Luck,
~Rick