Calling the authorities on neighbors ALWAYS backfires ... especially about animals. People are crazy when it comes to animals. Besides, if the animals are fed and watered and have adequate shelter there isn't much you can do except file a complaint. At which point they will roll their eyes, and show the inspector that the animals are "well" cared for and become angry at whoever they think turned them in. This is true even when animals are STARVING to death - many owners refuse to believe that their animals are receiving inadequate care. But people DO listen to people they care about and respect. The best way to help in a low grade neglect type case such as this, is to offer polite and sincere sounding advice to them. You might be surprised at how readily they take it especially since they are new to horse ownership.
Anyway sorry this was so long, but my advice is to make peace. Invite them to a barbecue or something and when they drive you crazy - find somewhere to vent. Then if you do see the need to call the authorities, you can look shocked and appalled when they vent about it to you
Also - I wouldn't worry about the mare unless she is in heat and if the colt hasn't been bred before he probably won't try too hard to reach her - of course it also depends on the colt. And maybe the crazy kids with their motor bikes aren't such a curse - I bet your mare is completely bomb proof now.