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A toddler is a person who toddles or “moves with short unsteady steps while learning to walk or being excessively drunk.”
I hope your young child is not drinking alcohol, so I think the first part of the definition applies. To me, a toddler can be anywhere from 12 months to 36 months. By the age of three, most children walk and run quite competently. At that point, you can just call them a child or a “little girl” or “little boy.”