I would start by sending an anonymous packet of info to them in the mail. Even though they are right next door it should go through the post office so they don't immediately suspect a neighbor, or at least which neighbor. This leaves room for you to do other things later without drawing attention to yourself. Here is a website with info you can print out and send them. It's about outside dogs and it gives links to other ways to provide better care for outside dogs by making them inside dogs. I would print out as much as you can and send it to them. If the dogs aren't spayed/neuterd, print out some info on that also and where some low cost places are in your area. If it doesn't look like they pay attention to the info, then do what you have to.

http://www.doggiedoor.com/outside.shtml

This link is a common sense approach about outside dogs:
http://www.critterhaven.org/outsidedog.htm

This is door hanger you can print out and put on their door or put it in the packet. I would navigate this site to find more info you can use.
http://www.dogsdeservebetter.com/doorhanger.html