means A young child, roughly one to three years old, who has recently learned to walk and does so with that distinctive unsteady gait.

from From the verb 'toddle,' meaning to walk with short, wobbly, tipping steps — first recorded in the late 16th to 17th century. 'Toddle' itself is of uncertain origin, possibly imitative of the rocking, teetering motion, and may be related to dialectal 'totter.' The noun 'toddler' for the small wobbler came later, around the early 19th century, naming the creature by the very walk that defines it.