After entering the territory of Kungenga Mine, start slowly moving south. In this location, there are a lot of enemies and you may be noticed, especially during the day (the same for the gunship, if you haven't destroyed it). Try to neutralize the opponents that can get in your way and stay away from the watchtowers (at night, there are searchlights patrolling the area).
Your current destination is the entrance to the cave located in the south-eastern part of the map. The entrance is shown on the picture above. There are at least two guards nearby and a single soldier on the watchtower. You can knock out the enemies or distract them by throwing an object or ordering D-Dog to bark. There is also a locked gate that you have to open and Big Boss will need some time to do it. When you're inside the mine, reach another gate. You will watch a cut-scene about meeting the children prisoners.