That's interesting. Resource guarding is not necessarily about dominance, so your dog may consider your child a higher-ranking pack member. In wolf packs, even baby wolves will aggressively protect their food.
The most basic way to prevent this is simply not to give your dog the valued object. A lot of people avoid bad behavior this way.
The other thing you can try is Take It / Leave It. Your child should practice this with you, though of course, you should always be safe and make sure no one gets bitten. Start with a lower-value object, maybe a toy. Have your dog "trade" the toy for something better like a treat. Tell the dog to "Take It" and "Leave It" as you switch him on and off the toy. When you are confident, try it with high value objects and see if your dog can exercise impulse control.
Good luck!
