Here are some options, depending on what you're trying to say:
For a young child:
* Toddler (for a child who is learning to walk)
* Preschooler (for a child who attends preschool)
* Child (a general term)
* Kid (a casual term)
* Tiny human (a playful term)
For a small child:
* Little one
* Small fry
* Wee one
* Sprout
For a child who is being mischievous:
* Rascal
* Imp
* Little devil
To describe a child's size:
* Short
* Petite
* Miniature
Please tell me more about the situation you're trying to describe, and I can give you a more specific suggestion!