Kids need a place that they can call their own. It can be a backyard shelter, no matter how crudely built, where a couple of friends can sit in and talk, or read comic books, or (nowadays) play games and text on their phones.
If your kids (girls too!) have a desire to build some kind of fort or hideout, by all means let them. Ideally, the less you help them, the better…let them discover the process themselves. Your task could be to provide a few tools, nails, and help transporting lumber, either used or new. Kids are surprisingly resilient; they won’t kill themselves or each other by building a clubhouse, and they will be doing something creative that will give them a deep sense of pride. Also, they won’t be crying that they’re bored, and you’ll know where they are!