I understand about that not wanting to be "that bitchy neighbor" but even if the dog can get use to you (and there are some things you can try with their help, which may help, but not necessarily) the dog is still going to threaten other people. If the dog goes off the deep end, goes past his threshold and attacks, a child, adult or another dog could be seriously maimed or killed. This is just too serious to not prioritize. These owners need to be responsible to keep their dog under control at all times before a tragedy happens. What would you tell yourself or some child's parents if you didn't say anything to the owners of the dog or an animal control officer and some child got attacked and ruined for life or killed?
They need to also realize that their dog is a HUGE liability and they're just inviting a law suit if they don't take control of their dog. As far as not being the one to tell them...so you're not disliked by them, I don't know what to tell you other than talking with an animal control officer and letting him/her do the explaining. If that doesn't help, if I were in your shoes, after explaining my concerns to the owners, they were uncooperative, I don't know that they'd be the kind of neighbors I'd care about liking me. I think it's important to explain politely, while maintaining some assertiveness or showing that you're serious. But anyhow...I'd start with the authorities and maybe they'll put the fear of God in these idiotic people.