Do you have sons?
Oh and she doesn't need to lift her shirt all her
shirts are made to show it off 
oh and she got her first pair of thongs last week.
I know you said in a later post that this is

, but there are lots of girls, of all ages, who do dress like this. And many go to school dressed like this.
I've raised three sons, and I can tell you what they think about 24/7 - girls, girls, girls, cars, girls, girls, girls, girls, girls...you get the picture.
I'm not saying it's right, but when a girl shows skin, a boy can't help but want to touch it. Maybe the girls are not saying "touch me" when that skin peaks out from under a shirt (or falls out of the front of a shirt!), but in talking to my boys, that is what the belief among most boys is. "Mom, she'd cover it up if I wasn't supposed to look, and feel, if I get a chance to!"
And if they see a tatoo there, well, a girl doesn't get a tatoo just so she can admire it, does she? (or a guy, for that matter)
Back to the original topic...the chaperone is there to assist the kid in making decisions, not to make a decision for them. Most parents of a 10 year old would have already said either "sure, honey, anything you want" or "not as long as you're living in MY house!". The 10 year old would have known what the reaction at home would be, and was willing to get the tatoo knowing what the outcome would be (either positive or negative).
I hope that the chaperone asked the kid (as a gentle reminder) what their parents would say, then he was obligated to let the kid buy whatever she wanted. Not his kid, not his money.