General terms:
* Kid: Informal and playful
* Little one: Affectionate and endearing
* Youngster: More formal than "kid"
* Offspring: Formal and biological
* Sprout: Informal and playful, suggesting growth
* Minor: Legal term for someone under a certain age
More specific terms:
* Infant: A baby
* Toddler: A child who is learning to walk
* Preteen: A child between 9 and 12 years old
* Teenager: A child between 13 and 19 years old
* Youth: A young person, often used in a more formal context
Literary terms:
* Whelp: Archaic term for a child
* Babe: A young child, often used poetically
* Brats: Informal and sometimes derogatory
The best word to use will depend on the specific situation.