Actually, this site is not intended for teenagers:
The Site, Application and Services are intended solely for persons who are 18 or older. Any access to or use of the Site, Application or Services by anyone under 18 is expressly prohibited. By accessing or using the Site, Application or Services you represent and warrant that you are 18 or older."
But teachers are offering courses for children and teenager and I also let my kids take lessons after having checked out the teacher.
There are sometimes users as young as 12 years looking for exchange partners but I think this should happen under parental guidance because there are many scammers here. I would first make sure that the potential partner is indeed a teenager of the same age and not an adult. I once posted a language partner request for my 12 year old son for German and French but got no replies. Now I found a language partner for him for Spanish as a language partner of mine has a son of the same age who is studying German. So, maybe you can look for a language partner who is a parent as well and then let your kids do exchange. It's a pity that your daughter isn't learning German, otherwise our kids could do language exchange.