I don't know how many kids it would take, but there are other ways of increasing the Parenting skill.
Possible spoiler:
Try dropping an adult on a child. They will tell the child a story, and if successful, they will improve in parenting. Also, when you try to get a couple "indoors", even if they aren't successful, they can still improve their parenting skills. That will only work on the villager that you drop on the other one.
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"Hope is the feeling you have that the feeling you have isn't permanent."
-Jean Kerr