It seems that the first girl, the pink-haired one survived her fall somehow. She gives birth to MCs son sometime just before the fire burned down the forest. They were in a cave and got trapped as the entrance caved-in.

They eventually starve due to lack of food and most eventually turn to cannibalism.

Also the fluffy animals eat them too causing them to mutate into the monsters. The first girl is shown to be slowly dying due to starvation and is about to be eaten by a cannibal when the MCs son (now older) tackles cannibal and check on his mom. Moms succumbs to hunger and dies causing her son to scream and go mad.