If you don't know which house the kid belongs in, the first thing I would do is to find out. Start knocking on doors and asking if it's their kid.
Now that you do know, the obvious thing to do is to introduce yourself to the neighbors and describe what their kid was doing. They're new to the neighborhood so you can pitch it as letting them know that there are a lot of passing cars at that point and kids should be aware of the danger.
If you don't know which house the kid belongs in, the first thing I would do is to find out. Start knocking on doors and asking if it's their kid.
Now that you do know, the obvious thing to do is to introduce yourself to the neighbors and describe what their kid was doing. They're new to the neighborhood so you can pitch it as letting them know that there are a lot of passing cars at that point and kids should be aware of the danger.