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I would think that a tampon would be easier for her than a cup at the beginning. However, it's really about her learning to insert ANYTHING into her vagina.
Since she is on a swim team, she will need to be able to use tampons or a cup. If I were you, I'd have pads and beginner's tampons (Playtex is a good choice) ready for her. Put vaseline on the tip of the tube for her and tell her how important it is to aim towards her lower back instead of straight up. Have her squat over a mirror first so that she can see where it is supposed to go. It's the beginning of understanding the makeup of her body. Girls and women aren't always very knowledgeable about that.
After she gets good at inserting the tampons, see about the Lunette cup. (The Diva is too big for an 11 year old, in my opinion.)
My mom was just awful about this stuff. She made my DAD take me to the gyn doctor before I could try tampons (a MAN, who was a real jerk) when I was 13. Not one of my mom's finest moments, I can tell you. It was my friends who taught me how to do it - my mom was absolutely no help. Thank goodness for my friends.