The mechanics will be finalized in the patches leading up to the pregnancy release. Basically, the mechanics I have in mind are the child is New Game+, and you play as it 18 years in the future, the previous player character becomes the family member lost to the Cave, and the journey begins again. This time searching for your parent, not your sister.
As to what the child looks like, there will be no children or art for children in the game. Physical traits will be a mix of the parents, (I'm still sorting this out) something like a human with the tames ears, wings, horns and tail. A crystal ball in the town will tell you what the child will look like as an adult, and that's the point at which you can choose New Game+ or not.
That is the current plan. But things change, and there is only so much I can do alone, so you will have to wait and see how it turns out.
Thanks for the continued interest <3