Two I used on my daughter who has no tattoos:
Girls generally love fashion, I told her tattoos are permanent, would she want to be forced to wear the same shirt for the rest of her life?
I also pointed out that when she was a youngster she would have thought it pretty cool to have one of the cartoon characters She-ra or the Smurfs tattooed on her, but at age 17 she would have been highly embarrassed by those same tattoo's. Likewise she will mature and her tastes will change nearly as much between the ages of 17 and 30 as they did between ages 5 and 17.
Good luck with the nieces and nephew.
Thanks....words of wisdom.