This is primarly due to them not being able to care for themselves. Caring for a baby and toddler's needs are distinct from that of other Sims. Since I've seen multiple people ask about caring for Sims 3 babies (both infants and toddlers), I thought I'd explain some of how the system works to Sims newbies. Imaginary Friends are a feature that is begun in the Toddler life stage, available only to Generations owners. Childhood and later ages are where you'll find the new gameplay. We didn't expect much for babies and toddlers, so this sparse content isn't much of a disappointment. There's a basic stroller, and one of the really big strollers. Another addition is that of Strollers, which will get your babies around for a walk. These rides are usually in parks, but this one is a super model that lights up with music, and can be turned on or off. Mom or Dad can let them get on the spring ride pictured here, but the adult must pick the interaction. There are now a couple of rides they can go on, and several new models of things that existed previously. The Sims 3 Generations brings a few more activities for Toddlers, but that's it. The Strollers and Spring Rides added to The Sims 3 with the Generations expansion Please remember some info or numbers may have changed.
This Sims 3 Guide was originally written for the PC and Mac versions of the game, but also will help owners of the console versions - PS3, Xbox 360 and Wii.
The Sims 3 Babies and Toddlers from Caring for a Baby to Toddler Potty Training Raising Kids in the Sims 3