I would tell them that when they become an adult they can get any tattoo they want but at 14 they are not mature enough to understand that a tattoo is there for the rest of your life. BTW I have multiple tattoos and don't regret any of them.
When I was 16 my mom took me to get one but she had to approve it. She wouldn't let my younger brother get one until he turned 18 and he would always say if she made him wait he was going to go crazy and cover himself in tattoos. He's 28 now and pretty much covered in them. So... be careful if they say they're going to go crazy with them when they turn 18!
I do have a 14yr old and she has asked for many ypes of things. Whilst I'm not against tattoos and piercings but I just don't want her to get a tattoo at such am early age but when she is old enough I won't have any issues accepting the things that she like