Argh, how awful! I have had a couple experiences like that, but the owners of the big dogs do not stand by and laugh while their dog terrorizes mine.
Thor is really tiny and I believe Missy is about half is size, so she must be microscopic. People do laugh at Thor sometimes, but it's generally in a friendly way. Latest comment I got - "does he run on batteries?"
I am glad you said something too, maybe the girl will think later about how "funny" it would be if her dog bit someone.
I do think it's really important that Thor gets outside and gets exercise, so we go to a lot of different parks, not necessarily "dog parks" per se. Usually I get a feel for how safe the park is. Sometimes the terrain is just too rugged for Thor. Even really tall grass can be hard for him.
Maybe if you look around, you'll find some better spots. The great thing about small ones is they don't need much room to have fun. I take Thor to a baseball field every so often and he has a ball. Other parks I take him to have a lot of room, and I'm just careful to keep him away from any big dogs that are running around.