Yes, it is safe. There are tampons made for teens, the very small light ones that she can start off with, and then can switch to the normal ones.
It is safe, it's a decision to be made based on comfort and anatomy, and stage of puberty. But tampons are small and are made to fit in the vaginal canal, and if you're old enough to have your period, you vagina should be able to hold a tampon, although it may hurt a little at first or cause a little discomfort.
Sara RN