I'm sorry to hear about your frustrations. I can imagine living in a large city there are not too many close, local choices for allowing your dog to go leash free.
Having a GSD has made me be more forward/assertive. So maybe next time you go to the dog park and an owner like the Pit owner is not intervening with their dog, you can kindly but firmly say to them to keep their dog away from yours, but I'd still plan on leaving. Having an 8year old in the park is inappropriate also. I've signed up for a few group dog events and usually the youngest age allowed is 10 yrs old.
Outside of the dog park I've witnessed unattentive parents ignore their kids due to talking on their cell phones in potentially dangerous situations. I live near the beach and one parent totally ignored her daughter walking on these slippery rocks that big waves were crashing over. No locals/surfers ever go where this kid was going.
I like what Anthony recommended, the one on one play dates, or see if there are any decent meetup dog groups in your area. Hope you have a better time during your next visit.