I'm not going to add anything of substantive value to the discussion because there's really no good answer because it's such an individual situation and a lot of great things have already been said. I just want to thank you for trying to take the right approach in handling this to guide / teach your child without becoming "that parent". It's a tough line to walk but there are so many parents out there who don't get it that it makes for a miserable experience.
What happens is the kid is so miserable that he / she doesn't want to be part of that sport / activity anymore but due to a lack of maturity, or scared of parent reaction, they don't have that discussion with their parents about why they don't want to be part of whatever. So they rebel by acting up hoping to get kicked off the team. Now they aren't the bad guy - the coach is. The focus of the parent anger is in the wrong place. I didn't see this as much when I was a head coach but I see it quite a bit now that I'm AD. I try to talk to these parents and kids to help them see the light but sometimes they just don't get it and it turns ugly. With the approach you're taking I find it difficult to think that your situation will get ugly. Yeah you're probably going to fight some but not like what I see in my experiences.
I'll say this, which is similar to what RJM posted, yesterday (Monday) during first period I have a sophomore softball player in my class. She comes in complaining about having to work out after school and asked what I thought about her not doing it. Well overall I don't really care because I'm not the coach and I'm going to be busy doing a million other things. So I told her she's a big girl and she can make whatever decision she feels is best but then I asked if she wanted to play in college (she has some talent but she still has to do some work) to which she said yes. I followed up with telling her I understand what she's feeling because everyone goes through that at some point but somewhere out there in the rest of the world there are several other girls who want that same roster spot at whatever school. They will end up working out today and they will work out on days they don't feel like working out. Do you want to be the odd man out of a roster spot because you didn't feel like working out? Each day you take off because you don't feel like doing it makes that next day a little easier to take off until you realize you're not working out at all.
Anyway - sorry for rambling all over the place with this post. I'll get back to doing other things.