Hey there,
I would recommend just talking to them about it. They have their reasons for being against tattoos, and hearing them out would make them see you as more responsible. Try and understand their objections to it (like how it's extremely permanent, how sometimes it's more difficult for people with tattoos to gain employment (in some situations)), and if you still want a tattoo, write out a letter to them that addresses all of that. That might work!
Feel free to
PM me if you ever need anything!
Kyra
