Unfortunately what also happens is that some people just like to see horses spook..they think it is funny. I don't know the number of times I have been in the upper paddock getting a horse when someone comes by and wails on their car horn or suddenly revs up a motorcycle just at that specific moment.
In this case 10-12 yr old kids should know better but I can see a lapse of judgement. They probably fled out of sheer fear rather than not wanting to help. My next question though is, why are 10-12 yr olds being given mountain bikes in the first place and being allowed to ride them on any type of trail without adult supervision?
One can hope the parents will come forward but facing huge vet and hospital bills, as well as the county fines, I can see them staying quiet.