that is a frustrating situation--have you spoken to the Mom at all? That can be tricky too--but it sure sounds like someone needs to be watching these kids!
I know my other neighbor lady has gotten right down to the nitty gritty with her, she doesn't beat around the bush or hold back...........she came right out and said "You need to be watching these kids better, one of them are going to get killed on that road, kidnapped, or something"