Those children are old enough to understand the concept of being sentenced to death by dehydration. Everyone knows thirst. This is not a complex issue. In fact, you'll find that young children easily understand abortion, too--it's murder. Only if brainwashed by narcissistic members of the Culture of Death into believing that it's okay to kill out of hedonistic convenience can anyone of any age support murder.
So they should be old enough to understand that there was no possible way that they would have actually been able to give Terri water.