The toddler phase is by far the most difficult phase in The Sims 3. All a baby needs to grow up well is to be attended to regularly. A toddler, on the other hand, has some very specific and important goals which need to be met. These include learning to walk, learning to talk, and being potty trained.
Learning to walk and talk are is simple because it can be done at any time. All it requires is for one of the parents to have one of the parents interact with the toddler and then select the learn to walk or learn to talk interactions. A skill bar with appear, just as when an adult is learning a skill, and when that skill bar is full the toddler will have learned how to walk or talk. Because this can be done at any time, these skills are pretty easy to teach.
Potty training is more difficult as it can only be taught when the toddler has to use the bathroom. That means the player will have to keep a close eye on when the toddler has to go. To make matters worse, parents left to their own devices seems absolutely terrible at this - micro-management is a must. It also takes several attempts to complete the potty training, and because potty training can not be done unless the toddler has to go, the training has to be spaced over several days.
In addition to all of this training, you'll still need to keep the toddler fed and entertained. Thankfully, toddlers are usually good about playing with any toys they are given. Make sure they play often, as the Having a Blast moodlet gives a massive +40 bonus to mood.