You have to keep aging up the kids and moving them out so you can have more babies. There are also rules about when you can age kids up; once a baby's 1st birthday notice pops up, you can age him up to toddler. Once the toddler achieves level 3 (or higher) in all 5 toddler skills, you can age them up to a child. Children need to become an A student to age them to teens. Teens need to be A students and, if you have parenthood, have 1 character value trait to be aged up to adult. Once they're adults you can move them out to make room for a new baby. That's why it's so challenging; it's not enough just to give birth to 100 babies, you have to follow the rules in bringing them to adulthood and moving them out of the house.