They'll never have any tragic accidents on the stairs (even though the animation can look a little scary), but their movement is so slow that they should always be picked up and moved between floors by an older Sim. Specifically regarding stairs, toddlers can navigate them eventually but such movement is painfully slow. A toddler with a low motor skill cannot navigate stairs by themselves and will need to be carried to different floors even if the Go Here order is given. For example, one of the Child Skills is Motor, which is a measure of how well they move in the world. A toddler's pie menu is simplified they're not having deep conversations or trait-specific actions with others, and they may be limited in their movements somewhat. Toddlers do not go to school but can be selected and commanded directly, and therefore adult Sims don't have to watch them too closely if you prefer to micromanage. Toddlers fill a time commitment gap between Babies and Children.
This trait will be lost upon aging up to a Child. When a baby is first aged up to a toddler, you are prompted to choose a Toddler Trait for them, which will significantly drive their AI when under Free Will.
