Babies- Three Days of raising the social bar which seems to fall the fastest of all, but that is easily cured by snuggling and playing with them. You can see
the baby's needs. Try to satisfy as many of them as you can before putting them to bed, and if you see the baby is about to wet himself within the next hour, don't bother sleeping until he has. Snuggle and feed the baby just before bed to help make sure they don't wake up in a bad mood. This is the only way your Sims will ever get a good night's sleep, as even having the baby in another
room will lead to frustrating midnight awakenings to a baby's cries.

Putting a teddy bear in the baby's inventory to sleep with will increase his mood, and possibly give your little one some extra lifetime reward points.

Toddlers- you buy Food Processor and a kiddie toilet.

Potty Training
Potty training is one of three skills that toddlers need to learn to develop correctly, allowing you to choose their traits when they age up. Potty training toilets are found in the bathroom and children tabs in buy mode. Once you've purchased one, you can click the potty with the parent to initiate potty training. The child has to be on the floor of the same level as the potty to do this. You'll need to do several sessions for the toddler to be potty trained. Empty the chair every other time they potty to prevent a mess. Once potty trained, your toddler can now go potty by himself. You only have to click the potty like you click the toilet for an adult, or just wait and let her go on her own when the need gets bad enough.

Sims 3 Parenting: Teaching a Toddler to walk. Now that toddlers can avoid dirtying their diapers, the hygiene bar will drop like normal. Using the change diaper action will restore hygiene to full.

Getting Good Meal Moodlets Through the High Chair
Forum member Arienrhod pointed out my lack of information on high chairs. If your Toddler's parents are decent cooks,
you can serve your Toddler leftovers if you have a food processor to convert it to mush for them.
Place the child in the high chair as opposed to clicking feed. That will then bring up the option to serve food.
Your Sim will use the food processor to make the food consumable by the toddler, and they'll get a positive meal moodlet for your effort.

Forum member Aiwe took the above information and used it to feed the toddler leftover Ambrosia. Simply cook it then place the Ambrosia in the fridge and it will be
available as leftovers to the Toddler. This reportedly does not reset their age bar,
but does give the week-long divine meal moodlet! Great for racking those lifetime happiness points up early in a Sim's lifetime.

Teaching Toddlers to Walk and Talk
To teach toddlers either of these life skills, simply click the toddler.
Both walking and talking are learned rather quickly.
If you wait for your toddler's social need to be low before you teach to talk and use the time when they're socially satisfied to teach them to walk,
you can satisfy a need and help them learn them at the same time, efficiently. You can probably get the toddler to
learn one of these skills each day if you can manage their needs. As mentioned above, Sims with the Family Oriented trait
will teach toddlers to walk and talk about twice as fast as normal Sims.
Sims 3 Parenting: Toddler playing with Peg Blocks

Teaching Toddlers Skills in The Sims 3 with Toys
Toddlers in the Sims 3 can get a jump start on life by learning skills to prepare them for childhood and adulthood. There are two known methods to teach toddlers skills. First, toys can help them. The peg box will teach them the logic skill, and the xylophone will teach them guitar skill. You can level up to three times with items like this. This is a wonderful option for an activity for toddlers while they're at home with the babysitter and the parents are at work. They can actively raise their skills and not waste all that time. They'll have a blast, to boot! Although they don't have wishes, Toddlers can get a head start on lifetime happiness points! Great for helping them to one day afford to buy the multitasker or observant lifetime reward. Adults and teenagers can also play with the toddler on these toys and raise the relationship bar.

Nine of the toddler books will raise skill as your child grows up. The only books that don't raise a
skill are those with Jimmy Sprocket in the title.
The yellow books (Finger Painting, Handprints of the Masters, and Don't Stay Within the Lines) will boost Painting skill when it's learned later in life (up to level 3). The green books (Bluish Eggs With a Side of Pastrami,
Frank I'm Not, and Oh the Destinations You'll Briefly Visit) will give a boost to the Writing skill up to level 3. Lastly, the pink books (Counting for Those Who Cannot,
Squares Are Not Triangles, and Being Smart For Fun and Profit) will boost one level of Logic.

Children
Keep the kid's mood up, do homework, and have the occasional friend over and your child will do fine in life. I'll add that it's definitely worth it to buy toys like the dollhouse when it comes up to earn the lifetime happiness points. In fact, childhood is a great time to rack up lifetime happiness and be able to purchase rewards very very early in life.

Teens- Make Sure They Have Good Grades.