approach her parents with style's great opener
"I could use an adults opinion on this one." [will work for sure because they feel responsible!!!]
"my cousin who's 8, really wants to get her ears pierced and her parents wouldn't let her. Now, she came to me knowing that her parents trust my judgement and asked me to tell them that it is really common for an 8 year old to pierce her ears, and convince her parents about that...
now, I really don't want to go against her parents wishes, but... I like that she can trust me with such things, that she sees me as a confidant and i don't want to ruin our bond so... what do you think i should do?"
doesn't matter what they will say, act as if you heard the most inspiring answer you have ever heard! and start conversing with all of them.
if done confidently it will work.
their daughter is the secondary target. one you open her parents and converse, the daughter will follow
Good luck
Mike.