Op coming back to you.
I think you are being very sensible by allowing her to meet her friends in the park with you sitting discreetly away, for how much longer will she tolerate that for though?
So for me the question would be do I want this for my dd?
Am I happy for her to go the park alone or with friends, for longer periods particularly as she gets older. What concerns do I have, and are they likely to happen in this park? What happens later in the evenings? As no doubt once she is used to going there she will continue into her teens.
Or do you want a different kind of independence for her? Could she go for hacks alone with friends instead for freedom, could she go shopping on a Saturday morning or out for lunch by herself? Can she do a hike around the area you live rather than stand in the park? Is there another way to achieve the same thing that is safer and more suitable for her?
You are at a crossroads, once you have decided it will be very hard to change your mind and stop the park visits later on.
She will almost certainly meet an established group of friends there, and your opportunity to help shape her decisions diminish.
It is your decision, and yours alone as she is only eleven years old and relies on you to keep her safe, she is too young to know the full consequences at this age (or she should be)
How comfortable do you feel about this now, in two years and five years time? Look further ahead to when she is fourteen and fifteen and will be coming home much later, be sure you are making the right decision for her, and for you. The decision is yours to make. It does not really matter what we think on MN. She is your child, and your responsibility.