Even for a mature young lady the attention of boys from "Competitor to Admirer" can be a scary and confusing situation for a young lady. And lets face it......she is still very young. Most girls do not really start appreciating boys until around 12 and those who do earlier often are the ones who are mimicking older siblings or friends.
In your daughters case this shows that she is a independent spirit and will not follow other girls trends...........which is a VERY good thing to know as a parent!
First off even though this is something she is going to have to deal with at some point in her life..........that does not mean she should have to deal with it now at this age. I would first recommend speaking to the coach. Let the coach know that the attention is making your daughter feel a little uncomfortable and at the age of these children this is not something any of them should have to face right now. Calmly ask the coach to speak with the children about keeping things friendly at all times. (The coach can do this in such a way that looks like a rule and not a individual issue).
Continue speaking with your daughter and let her know that many if not all of these boys are simply trying to mimic older brothers behavior and that is what they are trying to do. At ten they really have no real understanding of "Liking a girl" and most are simply trying to fit in with the other boys by following the groups actions. Explain to her that this is no reason to give up.........and that this is just another challenge for her to over come.
Something else she may want to think about, some boys may feel that if they express interest in her then she may become more self conscious on the field and thus not play as well........thus making their actions more a issue of them trying to compete on a different level. Since she is so athletic she may understand this a little more as well. Let her know these are "games" some players use to try and get under her skin. She may be more apt to handle it at that time if she looks at it as more a challenge then a real interest from the boys.
Above all do not make a big "stink" about this to her or anyone else........otherwise you may inadvertently cause her to think that there is something wrong with it all. By explaining to her that it is simply boys trying to act like they are older you are acknowledging the problem, explaining the reason, and lastly helping her learn how to deal with these advances (By telling her to tell them no).
I would also suggest that you may ask her to finish the season and if she still wants to quit then consider putting her on a girls only team if she really is willing to give it all up. (Though wait it out as it is better to try and teach her how to handle this herself first).
I wish I could tell you its going to get better.......but with school coming up and her age this will get more intense with boys at school showing her attention and other girls using peer pressure on her to "act" like them. Just keep it calm and teach her how to independently say no and in the long run she is less likely to have a issue with it at school and other activity's in the future.
Yes that would make it more difficult, I am sorry I did not realize these boys were on the other teams. I would make mention of this to the camp director. Keep in mind that the director does not want these kinds of issues either! I would simply explain that as a parent you are concerned about your child's emotional needs and ask what the policy is at the camp. (A little hinting that they should have a policy or a rule about this should encourage the director to mention it to the children during orientation). Of course that isn't going to stop all the boys.......but it may cause some of the boys to down play how hard they try for fear of getting in trouble.
As for what she should tell the boys, she needs something that will stop them dead in their tracks and end it. She should tell them in a no nonsense firm tone that she only wants to be friends. Have her practice it with you at home until she is comfortable saying it. Most boys are scared to begin with when they ask.......if she is firm enough it will be easier for them to get the point and drop it.