To have a toddler in The Sims 3, you can create one in Create-A-Sim, or you can have a Sim baby and wait 7 days for him or her to grow up, or you can force the growing up by buying a birthday cake, selecting Blow Out Candles, and choosing your baby Sim from the list that appears.
Proper Toddler Care
In addition to the training mentioned below, here are some things to know about how to take care of a Sim toddler. Toddlers can sleep in a crib, but not a bed, so don't get rid of that baby crib until your toddler has grown up into childhood. Also, toddlers can optionally eat in a high chair, but can also be fed while they are sitting on the floor.
Basic Toddler Training
Toddlers can be potty trained, taught to walk, and taught to talk during this life stage. To potty train a toddler Sim, you must buy a training potty, have the toddler on the floor in the room with the potty, and then have an adult Sim potty train the baby. Once a toddler has finished going potty, an adult Sim will need to empty the potty, because it's not connected to plumbing or anything so it can't be flushed away.
To teach a toddler to walk, the toddler needs to be on the ground, and an adult Sim needs to choose to teach the toddler to walk from the available interactions. The adult will stand the toddler up, then step back and tell the toddler to walk to him or her. Sometimes, the toddler succeeds, but sometimes, the Toddler will sit back down instead. The adult then laughs and tries again.
To teach a toddler to talk, just put the toddler on the floor and have an adult choose the option to teach him or her to talk. The adult will attempt to converse with the baby, and will make sounds of congratulations or frustration, depending on how well the toddler is doing.
Once the toddler has learned these skills, the toddler can then choose those options for himself or herself, not needing to go potty without an adult's help.
Advanced Toddler Training
To give your toddler a head start on adulthood, there are a few ways to give your toddler some skill training. The products in Buy Mode will display whether they train skills for toddlers. Also, there are books in the library and book store from which a toddler Sim can learn skills. Just leave the book on the floor with the toddler, and he or she might randomly choose to go read it.
If a toddler Sim learns a skill at this life stage, it won't show up in the Skills tab until the toddler has become a child and has trained another level of that skill.
It is important to have the toddler learn these skills, because if he or she does not, then the game will randomly choose traits for the toddler once he or she grows into childhood, and usually the random traits are bad ones.