I don’t know what it is about blanket tents and forts, but kids love them, even when they keep falling down. My only complaint is usually about getting my couch put back together, or about having some floor space again.
Kids can make blanket tents and forts however they like. Once they have the idea, you may not have to help a lot, depending on their age range. With my kids, the main issue I’ve found is that they don’t always understand that you can’t use the full width and length of the blanket as the roof, or that couch cushions need some support to keep standing.
Kitchen chairs make good fort supports too. I’ve shown my kids how clips can be used to hold the blankets in place, so they can make a larger space.
If your child’s bed has high posts, they may be able to make forts over their own beds. That can be fun for bedtime too, although a bit hot on many summer nights. Too little air flow. We use an old fitted sheet over my youngest daughter’s bed as a simple canopy, and blankets can be added to make it into a fort.
What happens once the fort is made is up to your kids. Once it’s stable, you can hope that they’ll keep busy playing for some time.