I would have to ask this: Have you or your spouse reprimanded her for going near your son in the past?  How old is the dog?  Most pits are curious dogs but if you’ve made the “mom noise” (I hope you know what that is) at them, not yelling or anything, they normally know from then on that behavior won’t be tolerated. Mine has learned the word “Stop”. He knows that whatever he’s doing at the time, he’ll stop and crouch down on the floor.  Usually his tail’s still going, but he’s waiting to see what we want him to do next.

If you’ve been hesitant to let her near your son when he was younger, she knows that and will steer clear of him, even if she doesn’t know why. I watched a video of a pit walking up to a baby. The dog went to lick the baby’s face and the person/owner with the camera made the “mom noise” at him. The dog immediately walked away from the baby. Every other scene after that the baby was walking up to the dog and the dog would walk away so as not to get in trouble. As long as the reprimand wasn’t too harsh, she should figure out pretty quick that you will allow her to be around the baby as long as she’s cautious.  

You probably don’t want the dog licking the baby’s face because she will think that’s acceptable, so if you’ve stopped that behavior before, she remembers. Though normally, dogs think of the children in the family as their pack mates, it’s most important that the pit understands all the humans rank above them.