You need to know the chemical composition of the paint. It sounds to me as if it might be a tar- or oil-based paint. Depending on the age of your car, you may be able to use either cellulose thinners or an acryl-based solvent. However, I would recommend you take the car (complete with paint splats) down to a spray shop and ask for advice.

You can easily passivate the paint on the road with solvents too - how does it react to turpentine? If it dissolves, then - problem solved. If it "curdles", then you have to change solvent. Alternatively, your city council might be able to help - it should be part of the responsibilities of the Road Services and Maintenance department.

However, the best solution is to kill the kids with a large axe to the head. Most State courts accept paint splatters to car paintwork insanity as a legitimate defence these days, but check with your attorney first to be sure before launching your decapitation foray.