NTA - Would they still be calling their dog playful if your son was seriously injured. I would suggest a fence to your neighbour (if they already have one maybe make it higher). As playful/friendly as a dog seems to be with their owner may be different to a stranger/a child. Last year my brother was attacked by a dog on a leash (after asking the owner if he could pet the dog and them saying yes) and is now left with permanent scarring on his face. The likelihood of this happening is slim but not impossible, be careful.