If your kid comes see you and she is desperate for a piercing and a tattoo. Number one understand these things are very different than they were when you grow up. Kids are largely all doing them so it is much more common place, so stop being shocked but rather ask why do you want it? Separate piercing and tattoo into two different categories. The piercing will not kill your kid, it is not a lifelong scar. You try to ask her to ascertain her motivations so she will understand that sometimes that to mutilate herself to get attention, to get into the crowd, whatever. It is a teaching moment. That piercing will not hurt her but do take the trip to a pediatrician cause they will talk about safety issues, some may raise if it is safe to pierce lower ear, move up in the ear, you much increase the risk of infection. So let her hear all that information, she may back-out at that point.
However, if you decide to go with it trade of with increase levels of responsibility, say honey if you can bring those grades up to B's you will earn yourself up a piercing. But sign a contract, one piercing is one piercing only for at least 6 months. Some kids get addicted to them, they just get one after the other. Tattoos are forever, as in you cannot un-ring the bell on that's it forever. So typically we say no, when you are 18, you have the power to do whatever you want but at 14,15,16 sorry sweetie I can't do this when it's permanent. You don't know who you are going to be in 2 years. You don't know if you are going to take on school or not. Trade off though, there is a Henna tattoos, it's a dye that they put on the skin that washes off. You can say "I tell you what, I've got a good compromise, I'll get you a Henna tattoo every month for the next 12 months if you'll just hold off on getting an illegal tattoo which could kill you. You go to illegal shop, dirty needle you can get a disease's". Kids go for that because they can change it all the time. So it really solve's the problem in their head, as whether they really want the skulls and cross bones on their arm forever.