1. Agree with her that the shorts might be a concern for these young ladies. Frame the choice of uniform as the issue. Be empathetic.
This is probably a line by her and not an actual concern of a majority of the players like she is trying to make it out. Sure some girls might have an issue, and I've coached a couple who did, but to agree with it's a lot of the girls is just fueling her fire on something that is probably not true.
Different situation but I had a parent complain about something that did not happen how she said it did. She told the AD "all the girls" so he in turn interviewed my entire team. It was found not one, not even the parents daughter thought how the parent did. However, her daughter told her mom she agreed and the "others do to". Have seen that happen more than a few times.
2. Suggest that she approach the AD about her uniform concerns.
Could be more than just the AD, uniforms are usually decided by the section, league, or state and the AD's just follow that rule when ordering. I've only seen one team that played against us that did not wear spandex and it was just the league had different policies. In my first school I coached the younger girls wore shorts but JV/Var HAD to wear spandex. The AD can however go to the meetings and bring it up as a concern but you won't see it changed any time soon.
3. State your goal to photograph this athletic contest and stay out of any uniform dispute.
Yup
4. Tell her that, at her request, any picture(s) of her daughter that she finds objectionable will be removed from the site. No questions asked. [Parents won't buy a picture they don't like anyway. Limit her influence to pictures of her child only. You may want to preemptively remove them.]
This can be a PITA to try and go through and find one girl through large galleries unless you know who it is. Sure remove them, for a fee. Make sure it's also known if it features another girl then it won't be removed. There is no obligation to remove them.
5. Explain that once the school administration chooses the uniform, they decide how the ladies will appear. Photos are only the messengers recording how the ladies present themselves in public.
Yup
6. Suggest that if there is a picture that the parents would like to purchase, but with a different crop, you would be happy to crop that photo differently.
Yup
As much as possible, appear to be on her side and redirect her objection to the administration. Let the AD be the 'bad guy' not you. After all, the issue is the uniform. Think cooperative, not adversarial. Stop taking pictures of her daughter in the future. Her line about "featured crotches" is a red flag. She appears to be looking for a fight. Don't let it be with you.
Nope, not when it's making someone else look bad. A photographer is there to capture an event, not fuel the fire of some nut job. Especially when she goes running to the AD saying the photog agrees with her and the AD is the one who decides if you ever shoot there again. She's definitely looking for a fight and jumping on her side when she's wrong is not the right approach. All you can do is say it's not your job about the uniforms, it's your job to take images of what is going on - that's it. Agreeing/disagreeing is not in your job description for this because it will cause problems. Fighting her she'll cause you problems, agreeing she'll cause you problems with others.