I agree that you can definitely tell the boys if you are there witnessing it. However, you can't keep on having to do that and the parents need to at least warn their boys, also.
I wouldn't talk to the parents. however. I suggest that you send a note (anonymously) letting them know in a VERY NONJUDGMENTAL way. And say something to the effect of:
"Dear neighbor, as we all know, many of our neighborhood kids (don't say "our" kids...because they will try to figure out how many parents on the street have children and will try to narrow it down to whom is likely to have sent this anonymous note) play in the alley. Although, for the most part, we are very lucky to have a great neighborhood that's very safe (keep things on a positive tone), I have noticed on a few occassions that when there are cars coming through the alley, your children are not afraid of of possibly getting run over. They actually would go darting for the cars, ride their bikes around moving cars, or roll balls under the cars, etc. I was very worried for their safety (voice of caring and concern - not judging). Please talk to your children (don't say.."talk to them about safety"..because that's already implied, otherwise you're being too specific and telling them how to parent) because we want yours & all of our neighborhood kids to continue to be safe and have LOTS of fun while they're out there." (make it a "community/were all in this together" kind of tone).
So do you see how that kind of tone, rather than "confronting" the parent will likely get a better reaction?
I would also try to stay away from words like they "always" this and that.
good luck.