I'm worried because my 12 year old just started her period and wants to go swimming. is it ok for her body/health, to wear a tampon at such a young age, what do you suggest?

If she's comfortable: It isn't uncommon for young women, even pre-teens, to use tampons. There are many newer, more comfortable products available. As long as she can insert a tampon with comfort (and maybe some lubricant like k-y jelly the first few times), a tampon should not harm her. Personally, i think it's more harmful for young women to be excluded from fun activities because of periods!