I agree that it looks trashy and sends a skeezy message, but the 10 year old should know better. Obviously her parents had never had discussions about what's appropriate for 10 year old girls. My mother would never let me wear anything inappropriate, and I would have known better than to get that tattoo.
What if the child wanted to buy something else? Should the chaperone be able to tell the child what she can and can't spend her money on? Maybe the chaperone didn't have a phone to call the girl's parents. Maybe the girl told the chaperone her parents wouldn't care. Obviously the chaperone thought it was acceptable because he let his own daughter get one. Different parents allow different things. No harm was done, so I don't think it's a big deal. If her parents were unhappy, they can have a talk with her about it and remove it.