Your teen will get a bill to sign just like the adults do, so you or she can add the tip there. I took advantage of this to make my daughter use her math skills to figure 20% (which shouldn't be hard at that age!) and sign the bill, so she knew what it was like to go through that process. I then showed up later to the spa to validate what she had signed. They were more than happy to tell me the total that had been charged to the room. (I'm willing to bet more than a few teens walk away without tipping....but hopefully I'm wrong about that.) I thought it was important for her to know how much it cost, how to tip, and hoped that she'd appreciate the gift that much more. I also thought it was important to doublecheck that the attendant had been properly tipped....."just in case"!