I know that it's scary when your dog reacts like that!
Adele is wary of kids- she has gotten better, but I know for her, she sees small kids as more of some kind of littermate than an authority figure. She is startled by their sudden movements and reacts to them kind of like she does an unfamiliar object. She sees my 4-year old nephew regularly (and will tolerate his affection) but she still doesn't trust him and has odd reactions to his behaviors. Like, if he jumps up from the couch or something she will jump away from him and she has growled at him once (they are always supervised together, by the way).
I'm not an expert but my advice is to try to slowly and carefully get him socialized to older children who are good with dogs first. (Maybe you could walk him next to a park with kids playing but maintain a safe distance so he doesn't feel uncomfortable and the kids don't come rushing over to pet him. This is basically what we did with Adele to get her used to the sound of children.). Does he bark at older kids, or just little ones? It's possible that small children seem like small animals to him. They do have higher voices and are pretty small, especially to an Akita.
Ok, one last bit here...if you really think he would try to harm a child then maybe you should enlist the help of a trainer. I hope I said something useful! Good luck to you and Zero.