OP - I apologize as I do not mean to be mean or unduly harsh - but I know it will probabl sound this way...again apologies in advance...
YOU have LET this dog KILL 78 of your and your children's chickens...After the first (or no more than 2nd time) I would not have bought anymore chickens until 1. I built a safer enclosure for the chickens and put up a 6 ft chainlink or welded wire fence on the chicken area and topped it with net - and 2. after the dog was dead. This way no more dead chickens, my children wouldn't be heartbroken YET AGAIN, and that dog would be dead to not be a problem, and then any future dogs would not be able to get into the fenced area with the chickens.
This is definately the neighbors fault for their dog is roaming and at large, and its just as much your fault for knowing this dog still lives next door and is at large unrestrained and still putting an all you can eat buffet out in your yard for him to have.
If he comes back again after your chickens, DO NOT SCARE HIM OFF. Stay inside and wait till your dad has the gun and let him shoot the dog while he's in the coop or yard killing chickens. Its more important at this stage for the DOG TO DIE than to lose 1 or 2 more chickens in 2-3 minutes while he loads his gun. Sorry but true. I'd rather lose 2 to kill him, than another 78 in the next few months to the dog!!!!!!!
Shoot it, trap it then shoot it, trap it then CO2 it to death, if you can get your hands on a metal toothed trap, use it even - inhumane I know but atleast it wont be going anywhere for you to be able to shoot it!!
If you can't kill the dog - or safeguard your chickens you need to not get anymore chickens because your just feeding this dog's blood lust and your breaking those kids' hearts just allowing this dog to continue shamelessly killing.
Sorry but thats the way I see it.
Sorry for your losses, but you need to be proactive not reactive and be done with that darn dog.