Talk to her, face to face. You don't want it to look like a dad that swooped in and took a team away from the young ladies.
I agree. I would suggest you give the HC some honest feedback, and help her figure this out. At 22, it is tough if this is your first time dealing with parents and kids as the HC. You push her aside now, and the organization will lose her for good. Help her succeed, and you will have a great coach, maybe for many years.
If you are dead set on taking over, it will be the beginning of the end for your team. You've already sown the seeds of discontent among parents, and if you don't find a way to be "fair" to all of the players and be successful right away (ie win games), you will be seen as an overbearing disgruntled daddy-baller by parents who think you are not treating their kid "fairly".
I have to admit, when these situations occur on other teams/organizations, it's fun to watch the parental drama, but I always feel badly for the players.