Definitely keep contacting the police or see if there is a park ranger. If the person you talk to isn't helpful go above them. Keep at it.
One thing I noticed is that the bigger dogs run up to you and your group and jump on you. Except for the pitt, it sounds like they aren't trying to hurt you or your dogs. Although that's still not acceptable at least they aren't trying to attack. It's curiosity. Try to remain calm so that both your dogs and theirs don't react.
Just a little bit of info. Some dobermans, not all, will appear to be charging towards you. Often they are just very curious and run right up to the person and then come to a stop within inches. It can be very intimidating and scarey. Every once in a while my dobe would get loose and that is exactly what she would do to someone walking down the street. She just wanted to meet them. She thought everyone was a friend. I was so embarrassed until I got her to stop doing that. Obviously you can't train their dog but what you can do is turn your back on them when they get up to you and ignore them. Do not look at them, do not say anything to them, do not acknowledge them. They don't like it and get bored. We did this in training to get them to stop jumping up.
Another thing you could try doing , if possible, is when you get to the park and before you take your dogs out of the car, go up and introduce yourself. Talk to these people and ask about their dogs. Complement their dogs. Then tell them that you just wanted them to know your niece is terrified of big dogs. Would they mind leashing or distracting their dogs just until you get out further in the park. I know it seems obvious to us but they may not realize or see their dogs as a nuisance. They see those big dogs as their "babies" and think of them as harmless. I was guilty of making the same assumptions and would roll my eyes when people with little dogs would walk in the opposite direction. NOW, I know better but it's all perspective. (I attached a picture of my two.) I see both of them as my "little babies" even though there is huge difference in size.
Last but not least their is spray you can get at pet stores that is specifically for deterring dogs. It's harmless to them but gets them to go away. That way if you have to use it and the owners complain you can show them the product and then tell them you weren't sure what they would do so you were taking precautionary measures and gently remind them there is a leash law. Would they mind leashing their dogs until you go by.
However, if you really feel you or your dogs could be hurt I wouldn't take the chance of continuing to walk in that area. You can't change the behaviors of others, only your own. Find another area that you like to walk in and change your routine. I know it doesn't sound fair but at least you will know you and your babies will be safe. Take it from someone who lost her baby. I'd do ANYTHING to keep her safe. It's just not worth the risk.
Like the others said you have a right to use that park but they do too. Unfortunately you can't make them follow the law and keep the dogs on leashes.
Hopefully something I've written can be of use and will work. Good luck!
Mouse's Mommy
