Child
The Child life stage exists in The Sims, The Sims 2 and The Sims 3.
The Sims
Childhood in The Sims can either be the state that a Sim was created in, or attained after 72 game hours as a baby. Children do not grow older, and can't gain skill points. Children attend school every day to maintain good grades (they can also study from a bookcase, which prevents them from losing grades when they miss school). Child Sims whose grades remain at F for too long are sent to military school and are never seen again. Therefore, it is essential that Sims go to school every day, on time, and in a good mood to ensure their grades remain good (and to ensure that they keep getting $100 from their grandparents). The children of the neighborhood can play with one another, and another (sometimes unwanted) child will sometimes drop by your house to play with your kids. If they stay into the night, their parents will come and take them home.
It is possible for children to function well without any adults present in the house, however, they would be relying on good grades to keep the money flowing and ordering pizza over the phone to eat or having Servo installed to cook the food. Children can also care for a baby.
With the Makin' Magic expansion, magical children has access to the spell Age of Instant which with permanetly turn them into adults. This is the only way for a child to become an adult without the use of a hacked item or other cheat.
The Sims 2
Child
Children- full of boundless energy
Childhood is the third life stage in The Sims 2, and it lasts for 8 days. Children will still have the same Grow up aspiration they had as toddlers, but the wants and fears are different. For the first time, making friends is important, as is being taught to study by an older relative. The school day for kids is 9AM to 3PM, Monday to Friday. Like the original, children whose grades are consistently poor will be removed from the family by the Social Worker (who can also take them away if their hunger or social motives are too low or if they are too hot or cold), but can later be adopted by other families.
Unlike the original, The Sims 2 children can't live alone without an adult present, but can earn skill points. They can ask for (or be offered) lessons in all the original career reward items, and even use some of them independently. Children can't cook unless they use the toy oven, but they can still study cooking from a bookcase or the Yummy Channel. The Seasons and newer expansion packs allows putting away and retrieving leftovers, making Sim kids a little less dependent on adults for proper food.
The Sims 3
In The Sims 3, Child is confirmed the third life stage. Children can have three traits and take _ days to progress to the next life stage: Teen (Child Sims are about 5 - 12 years old)
As a child your Sims will not have a lifetime wish but will be free to build skill levels, pursue hobbies and develop friendships that will affect later life.