In the US, its broadly defined as the physical, psychological or emotional mistreatment of someone under the age of 18. Its more practically defined as any act or failure to act that leads to the harm of a child, so it can include neglect.
The legal definition is long so I'll shorten it. The legal definition states that child cruelty is whipping them unnecessarily, or by neglecting to provide for them those necessaries which their helpless condition requires, neglecting to provide adequate food and water, medical care and exposing them to inclement weather.