Tell the parents that you don't want trouble but that you have a right to protect your property and pets. Tell them the next time, and any time in the future that you see anyone harassing or otherwise molesting your birds, you will call the police. And then, do it.
Parents have a hard time thinking their precious little children could possibly be the culprits, so if possible, take photographs of them if they cause trouble again. This will help the police do their job with the parents, as well.
I'd be as upset as you are. Something is very wrong with kids who would do this kind of thing. But so many parents are lazy or absent (working) parents these days, with good parenting taking second place to money or whatever.
Frankly, I think I'd be considering a hidden security camera, so I'd have a record, if anything happened again. This would also be helpful for identifying neighbors' dogs, etc. Dog parents have the same troubles imagining their precious pooch being a culprit. Other peoples' pets can do a lot of harm in a very short amount of time, so do take precautions.
I think secure fencing is a very good idea. If you absolutely can't afford to fence your whole property (which often does increase your property value), you might consider an electric fence, which can easily be moved around your property, allowing your chickens new spaces in which to forage. Check out Premier and/or Kencove portable poultry fencing (sometimes referred to as sheep fencing). You can find them online. Or you can learn online how to electrify most any fence.
Electric fences won't harm the kids or dogs, but they'll get a nasty zap. As will other mammalian predators, like raccoons.
Problem is, those kids sound like mean-spirited ones, and they can get creative--lobbing rocks at your chickens or poisoned food, etc. So stay watchful.
And good luck!
