Sean is spot on the dog must be "under control" that doesn't mean on a lead.Secondly a farmer must catch the dog "in the act" of worrying stock.Thirdly,there is no trespass law in this country,you can only be prosecuted for causing damage by trespass (fences,crops,etc).
As a few have said,if it were me out with my dogs and children,id have beat f*ck out of him,and then contacted the police.My kids are the most precious thing in the world to me,and discharging a firearm near my children is provocation enough for a beating.
I would contact the police and follow this matter up.