Thanks, everyone, for your thoughtful replies!!!!! Very much appreciated. I'm divided now over talking to the parents, but if it looks like they're home tomorrow I may knock on the door and try to present it as a safety issue. I agree that I need to watch my words re: the biting issue, as even mentioning the risk (which is a risk with any dog, realistically, if you provoke it enough) can put me at a disadvantage as somehow admitting that my dogs are vicious. This is California. People live for legal loopholes.
I did consider trying to show the boy how to approach the dogs properly, minus the scooter, but I'm afraid I just can't trust him to listen. He seems aggressive and a bit "off", if you know what I mean. And when I don't feel comfortable with someone, I usually err on the side of caution. My dogs, my prerogative.
Mr. Gemini was just out with the dogs this a.m. but didn't see the boy or the scooter. I will also ask our dog walker if she's met this kid before. If I do go knock on the door, I'd like someone else to be with me so I have a witness and also for back-up if the parents go ballistic on me.
Thanks again for all your responses and input. It really meant a lot to hear the support and advice. I'll let you know how it turns out.