Nanny
From The Sims Wiki, a database for The Sims series that anyone can edit
Nannies are NPCs that take care of your Sims' children while they are at work during the day. Nannies can be hired at any time by phones through Call... > Service > Nanny. Your Sim can choose to hire the nanny on a one-time only basis, or they can have the nanny 'track their schedules' (their work hours), therefore hiring her during the times they are at work.
The nanny will take care of your Sims' children (bathing, feeding, changing, and interacting with toddlers and babies, and interacting with children), as well as tidy up the house by doing things like emptying the potty and making the beds. They can be fired (End Services) at any time, and a new nanny can be hired to replace them.
In The Sims 2, nannies are always female elders, and wear a conservative white blouse, gray button-up sweater, long plaid skirt, and bowling shoes. They can have any hairstyle, skin color, and personality. A nanny can be interacted with, and therefore married, asked to move in, or killed off.
In The Sims 3, a nanny is changed to a babysitter. A random teen from the town will come to take care of the baby/toddler when you call for the service. Babysitters can be male or female. They are just usual Sims that come to help for a fee, so you can interact with them just like you can with other Sims. If a certain babysitter keeps showing up for a few days and has a lot of interaction with the toddler, their friendship will continue when the toddler grows into childhood.
[edit] The Nanny Necessity
Should a Sim need to go downtown, or to work and is the only adult, elder or teen Sim on the lot with the baby/toddler/child, a nanny must be called or the social worker will come for the offspring instead.
A nanny can be called up to come over once or to track the schedule of an adult to see when the child will be left alone. However, this schedule stays the same if the Sim is promoted, so you will need to call the nanny again if this is the case.
Generally, once a Sim has re-appeared, the nanny will leave, but sometimes they need a bit of persuasion.
[edit] Nanny Problems
Some nannies are more on-task than others, and careless nannies are apt to make meals and possibly start a fire, wet themselves, and neglect the children. There's also nothing to stop them stealing from the Money tree except locking it in a secure room that only a certain sim can enter.
Such feats of child cruelty include repeatedly picking a baby up and putting it back or on the floor or putting a toddler in and out of a high chair, and staring blankly at it. Others include making a TV dinner for a toddler that can't eat it, then walking off to pick the toddler up and down again. Sometimes, they can do something so silly, that the child/children can be taken away.
Nannies can be also quite glitchy. They can become stuck on lots, multiply or simply refuse to leave even if the van shows up. In this case, the simplest solution is moveobjects cheat. The Nannies are probably so glitchy because they are not meant to be on the active lot, as the parents of the children are visiting a community lot.
In The Sims 3, babysitters were known to take the child when they are going away thus making you lose your child. This has now been fixed[1] with the 1.2.7 patch released in June 2009.
