The bark is highly aggressive and verging on scary for my child.
This may not be aggressive (not trying to minimise your fears, but to reassure!). There is an old saying that a barking dog won't bite. Barking isn't usually a sign of aggression, but used as a warning or alarm, a greeting, an expression of excitement, an invitation to play etc.
A dog that is intending to attack will sometimes give a low, guttural, bark but it will more usually growl or snarl. Body language is also a good indication of a dog's intention: aggression is indicated by a dog going low ready to leap or lunge, and in a prick-eared breed like the GSD, the ears are likely to incline backwards - ears forwards are a sign of interest and curiosity.
My late mother, who was something of a dog whisperer and knew more about dogs than anyone I've ever known, always used to say that if they're not showing teeth, they're very unlikely to bite.
She also taught me the dangerous dog drill: stand sideways on to the dog, fold your arms and don't make eye contact with it.
I complained to the council about a year ago but there’s not much they can do since they own.
I'd try again. Councils are required to have a dog warden, to deal with strays etc. As your front garden is shared, it counts as public and they have a duty to deal with out-of-control, dangerous dogs in a public place. From their response, it sounds as though they were treating it as a noise nuisance matter.
Make sure you explain that it's an aggressive, out of control dog in a public place.
If they still won't get involved, then report to the police.
If it was me, I'd work on befriending the dog but this would be difficult to do without the co-operation of the owner and it sounds as though things have gone too far for that.
I feel sorry for the dog, but the owners are plainly arseholes.
Out of interest, how does the postman manage to deliver the mail? They can refuse to do deliveries if there's a dangerous dog loose, and then you have to go to the post office to pick up your post.