I highly recommend considering a reusable menstrual cup. There are all different sizes, some of them very small, and they are great for swimming. These cups are made of reusable silicone that catches menstrual flow and is inserted like a tampon. Also like a tampon, they are easy to use once you get the hang of it.
The Diva cups (the only brand I have been able to find in stores in the US) are actually on the larger end of the spectrum, and I'd recommend that your daughter consider a smaller cup. I like the Lady Cup (it comes in lots of fun colors- google ladycup) and both the small and large cups are smaller than the Diva cup. I ordered mine online, but have heard that ebay is a great way to get the cups for a better price.
Also, this message board has lots of info on all different kinds of cups. Here are some links (the last one has a bit about a teenage daughter using the cup too):
Good luck! I wish I'd known about these years ago.