Rocks

Second post of the day...so not long after I finished the first post today, I noticed several neighbor boys sitting next door/across the street throwing rocks out of our neighbor's landscaping around his tree. The rocks therefore are bouncing across the street toward my house and into the yard and obviously littering the street with tons of rocks.
Enough was enough so I went out and told the boys to stop throwing rocks. The oldest child was maybe 8 or 9, the youngest was 3. Two of the boys were pretty polite and I'm not sure if they were throwing rocks at any point or not...they even helped pick them up. The other two boys scattered down the street without picking up any. I made sure to let them know as they took off that I would be talking to their parents.
I walked down the street (they live on a different street than we do) and knocked on the front door two times--no answer. I walked to the side/back of the house and saw a boy. I asked him if his mom or dad were home. His dad was in the garage. I told him what the kids were doing and he was pretty decent about it--I suppose. Mom came out of the house as well.
I should point out that these people fail to watch their kids very well most of the time. The other day I saw the child that was probably 3 or 4, take off running down the street pulling his under aged 2 sister in a ride toy. He was like 2 blocks away when finally his mother came out to the yard and called him and told him he couldn't take his sister after all she's not old enough to be that far away from home... (she probably will be old enough in a few months based on how much they all run around).
Anyway, what would you do? Would you want to know if your kids were throwing rocks everywhere? Would you tell the parents? What would you say? Yikes!