LeeLee is exactly right. Your dog thinks that the kids are beneath him in the pack pecking order, so if they try to take his bone (or if he thinks they're going to take it), he's going to bite or growl.
Your dog needs to be properly trained to help learn and reinforce his place in the pack (your family). It makes a world of difference.
I also recommend reading "Good Dogs, Great Owners" by Brian Kilcommons. It's truly one of the best books out there on dog behavior and training. And don't go to the pet store for training by someone who makes barely above minimum wage. Go to a local facility whose whole business is working with and training dogs. Do your research and find one that has good reviews.
ETA: I couldn't disagree more with Gamma. Sorry Gamma.
You should be able to reach into your dog's bowl ANYTIME and not worry about what the dog is going to do, because the dog should show deference to any human. Even a baby. It takes time and training to do this. Starts from puppyhood. Pulling the bowl away. Putting fingers in the bowl while he's eating. Taking a toy or bone away and giving it back.
Teaching your dog deference is the most important part of training, IMHO. It's the reason your dog listens to you.
Our dog was a year old when we brought her home, and was food aggressive. I took her to training classes to work on her deference and confidence (a dog who knows the rules is like a child....consistency brings confidence and good behavior). We worked on deference at home and trained at least a half hour a day after we exercised. All the while, I worked on her with the food. Taking it back, putting my hands in, even putting my hands in her mouth. Today....not a bit of food aggression. I put down the bowl, and she sits and waits for me to give her permission to eat. When I say "okay," she may eat. But if I lean down, because she's been taught to defer to me, she'll look up at me to "ask" what to do.
All done with training.