Your Child is the born after you marry a bachelorette and maintaining high affection with her for a month. He is unique in that he may be shown to people (Gotz and Kent) for affection bonus (becoming a renewable item for affection) and can be shown many different animals to increase his own affection (thereby not needing to give him a gift and still receive a lofty affection bonus).
Your baby has three stages: crib, crawling, and toddler. He is featured in a few cut-scenes involving him getting sick, and running away from home when he gets old enough.
The following animals increase his affection while he is in the crib or crawling: