OP here - I spoke to the mom this morning. The parents plan to drive the girls up there and drop them off about 2pm. They are not going to stay in the immediate area the whole time but will head back over to the mall near the park for dinner and a movie and will pick them up about 930pm. I'm mostly ok with that plan and have decided to let my DD go. The mom did offer that if any of the parents weren't comfortable with that, she and her DH would stay in the park the entire time.
Reasons for letting her go (letting go!):
My DD is responsible and on the cautious side most of the time.
I will have her text me throughout the day (although I can probably follow along with their day on Instagram, lol).
The other girls that are going are good kids.
We've been to that park a ton of times and she knows it well.
She'll have her phone (it's practically glued to her hand) and her back up charger pack.
She has earned my trust.
Sounds like a fantastic, well thought out plan. I hope your daughter and her friends have an awesome time!