Canadian police take the harming of pets very seriously. we had a similar issue arise here in my town, the 2 offenders are now facing court proceedings. they thought it would be funny to tie someone's beloved pet to the back of an ATV and drag it. an area resident saw what transpired and called the police. they are in a world of trouble. I would call the police and also if there is animal rescue group of any sort in your area put out a call to them and advice them about the cruelty these little buggers are handing out to animals that cross their path chances are your animals aren't the only ones being hurt, and that way they will have a heads up and you will have a group to back you up if a future issue arises. Do not hesitate to get the authorities involved.
Edited to say I would also call the Childrens Aid society if I were you, as there is an obvious reason the kids are the way they are and there is a huge lack or supervision too on the mothers behalf. CAS won't even ask your name now, its quite confidential, and you can tell them you prefer to remain anonymous because of the type of people they are, straight and simple.