Start from picking up the kid that cannot walk on their own. Go back to the place where there are two corridors in the cave and choose the path that you haven't been to yet. You will reach the area south from Kungenga Mine. Your last task is to escort the children to the evacuation point located south-west from here.
To reach the exlfitration point, you need to follow the path shown by the game. It leads south first and then it turns south-west. Of course, you won't make it to your destination without encountering some trouble. There are a few camps on your way and at least two guards in each one of them.
Every time you approach an enemy camp with soldiers (use D-Dog to learn their location), you should do two things - put down the wounded kid and order the rest of the children to wait (select Wait in the drop down menu). Only then start clearing the area. Try to stay unnoticed, because you may lose the chance to complete one of the optional objectives (avoiding raising the alarm). After you secure a given area, you can pick up the wounded kid and order the res to start running (select Go).
Start heading towards the evacuation point. If you got rid of the snipers near guard place number 13 earlier, the task will be easier now. Otherwise, stick to the rocks on the right. When the gunship lands, place the wounded kid inside. Start picking up other children and place them on board. At the end, join them in the helicopter.
Note - if you haven't shot down the enemy gunship, wait until it moves to another part of the map before you start evacuating the children. Its crew may see you and open fire, making you fail the mission (destroying your helicopter or killing the children).