I'm involved in a church holiday club that welcomes about 150 primary aged children for a week each summer - we don't charge but donations are welcome.
We couldn't do what we do without our teenage volunteers, who are there for 4.5 - 5 hours per day, with no more than a toilet break. I think we have about 50 for the week, and several in the weeks beforehand making scenery, etc.
Of course, they don't get paid as they are volunteers, nor do the adult volunteers get paid. I can comment on the ethics of this if the customers are paying handsomely, or in any expectation that it's what you do if you are a member of the dance school.
On the point of ratios, insurance, safeguarding and risk assessment, these all apply. Teenage volunteers add to the ratio headache, so we need to make sure we have at least 4 adults just to cover the teenagers. There are risk assessments for everything (this is my role, and it takes a good couple of weeks to do them properly), and the insurance company require this. The teenagers also have to do safeguarding training, which is another 3 hours of their time. The need for these things do not distinguish between children, adult volunteers or employed people. We also have to apply the Church of England's Safer Recruitment, which involves extra work when the teenager is not one of ours.
We do lots of references throughout the year for these teenagers, but thankfully they are not transactional. Their main reward is that they enjoy doing it, and they know that they are loved and appreciated.
For some families, the parents are really grateful for their teenager to be well occupied for a week, instead of doom scrolling on their phones, costing you extra money, or being a nuisance in town.