YANBU. A car park is not a play area. Period. It is very dangerous place.
You need to report them to whoever own the flats and car park. Send them a strongly worded letter stating that they have failed to make the area safe and it is being used by children WHO LIVE THERE as a play area. Advise them that you and/or others have had a near miss already. If you hit one of the children, you will hold the car park jointly liable for all injuries to the child, your vehicle, your person including psychiatric damages. Advise them that if no action is taken immediately, you will be billing them for the damages to your vehicle, enclose pictures and an estimate for the damage to let them see that this is a serious matter which has already escalated and it needs to be dealt with appropriately and immediately. If you can get any photos or videos of the kids in question, even better. Enclose a copy or email a copy of he video/audio file. I would push for CCTV, notices and letters to all tenants/residents. Suggest some kind of barrier. The owners of the car park will likely warn the tenants/residents and perhaps it might stop that way. Unlikely knowing kids, but possible.
I would get a front and rear facing dashcam and install it immediately. Any further contact with the kids family I would do in writing.
If you can prove who damaged your car, you should be making a claim on your insurance and providing the parents contact details and let the insurers take it up with them. They're supposed to keep their children under control at all times so are liable for any damages their children cause.
Really the parents are guilty of neglect. At the minimum, there are major safe guarding issues. If you've warned them of the obvious danger and they are still choosing to ignore you and the others, perhaps it's time somebody spoke to them about their children in a formal capacity. What they're doing is unnaceptable.
Ultimately it falls to, whether or not you want a dead or seriously injured child on your conscience. Somebody may be reversing out one day while the child is down on the floor fishing around for their ball, they won't see them and the rest will be history.
The above sounds like overkill but it's critically important to protect any child from future injury and yourself if you are the driver of the car.