When do you step in?

So I woke up Saturday morning and while down stairs the dogs ran outside and started barking. Our dogs aren't normally allowed to bark unless there is someone or something around the house. So I glanced out the back door and saw two kids in their upstairs window looking out. Not only were they looking out they were spraying Windex through the screen at their dog below. Soon the Windex bottle became a gun and he was shooting my dogs (making the noise and all). I didn't think it was a big deal (minus the kid having Windex) he's just preparing to enter school and is getting a little target practice in. Right? The big problem came when he stood up in the window sill and pressed his face against the screen as his sister was banging on the bottom! Then I freaked. This kid was going to plummet 2 stories and hit dirt, concrete, or furniture below him! I sat and watched (and took pictures) waiting with shoes on for him to fall so I would have to jump the wall and help this little guy out. I know I did some really dumb stuff when I was a kid but at that time, if a neighbor yelled at you and you listened. It was okay to yell at other kids. You can't do that now. Thankfully his mom walked in the room yelled at him paddle his butt (while he was still in the window!) and cleaned the mess. So the question is: When do you (a) yell at a kid to get out of the window sill (especially the screen side!) or (b) walk over to the house to tell his parents? and then what do you say, "hey I live kitty-corner to you and I've been watching your kids play with Windex and bang on the screen up stairs, I even have pictures to prove it"? I could see the next step to that...Emily Brashear "Registered Sex Offender"! Anyway, here's the little guy and his sister when they were just starting out on their "cleaning day".