I don't have a clue why it has started at 8 months
There's part of the puzzle right there. He is in canine adolescence and is flexing some "teenage" muscle. Since there are children involved, I really suggest that you seek out a qualified behaviourist if you can.
The snapping might have been unprovoked in your view, but in his (the dog) view something did happen to provoke, and without knowing what the details are surrounding these incidents, no one can really comment on it. A behaviourist could observe you and your family at home with the dog, and possibly witness such an incident, and make recommendations.
If that is not possible, I would get him in a
good training class with a knowledgable trainer who can make recommendations on how to use the excercises learned in class in these situations at home.
Also google NILIF, a training method based on Nothing In Life Is Free, where the dog has requirements to meet before he can do or have what he wants, such as doing a sit or down before being fed or going out the door. It doesn't even have to be something so "stern", you could have him fetch a ball or roll over or do a trick; the point being that the terms are set by humans not him.
I really do feel that when you have a potential biting situation and feel unequipped to cope that you seek out a behaviourist or trainer. Even small steps such as an obedience class will help get the situation in hand.