Well, what would you do? I have not been in a situation such as this, but I know it helps to talk about it and plan in your head. If you tell yourself over and over that you will react a certain way when it happens, than you are probably more likely to react that way if it does happen.
I think I would back the child into the corner so he could not run. Then I would bring the pistol into plain view at the alert position (muzzle to the dirt at 45 degrees with both hands on pistol). Next, I would give loud verbal commands for the BGs to stop advancing. If they do not, I would escalate my level of force. 21 feet is considered the MINIMUM safe distance for a threat with a knife. Multiple men, to me, makes 50 feet too close.
If they are brandishing weapons like you say, I would not hesitate draw my sidearm for one second!