There are health risks for any girl, of any age, who wears a thong. The most common health risk is yeast infections, which can be caused by wearing tight-fitting clothes like thongs that don't allow the skin to "breath." Thongs also increase the chances of getting a UTI (urinary tract infection) because they can move bacteria from the rectal area to the urethra.
Other health risks associated with wearing thongs include skin irritation, rashes, and hemorrhoids. So while there isn't necessarily one specific health risk that's unique to 10 year old girls who wear thongs, there are certainly a number of risks that all young girls should be aware of before making the decision to wear a thong.
It is ultimately up to you as the parent to decide if it is okay for your daughter to have a thong. Make sure you discuss the potential health risks with her, and remind her that it is ok to make her own decisions as long as she is informed and comfortable with them. If you decide that it is okay for her to wear a thong, make sure she has access to comfortable, breathable thongs and has the knowledge to practice good hygiene. Above all, make sure she is aware of how her choices may be perceived and that she is comfortable with those implications.
At the end of the day, it's important to have an open and honest discussion about this decision with your daughter and ensure that she is informed and comfortable with whatever choice she makes.
Ultimately, it's up to you as the parent to decide what you believe is best for your daughter, but it's important that she be involved in the decision-making process. Good luck!