Of course, that, and probably the majority of this thread, would need a better definition of "child". An eight year old might be selfish and reckless, but is also old enough to understand it's wrong and choose not to. A two year old will beat you to death with the button for not giving them the toy immediately.
I think that might be a problem at hand with the saying because at what point do you say "That Is A Child." Some will say that the age of 5 is a limit and when they reach 6 and older they are considered a "Kid". On the other hand some might say until they reach 11 and 12 years old they are considered children. Personally my own definition of a child would be a creature/being/human that is below the age of 7 and does not have the concept grasp of right and wrong. There are some who are very smart at 6 and 7 years old thus they could not be called children instead being called "Big Boys/Girls" however that maybe another point of judgment that varies from person to person.
Personally I do find children to be quite cruel because as noted they don't have a true grasp of consequences in regard to their actions. As a result they perceive things as "Fun For Me" or "Painful To Me" and either want to do it nor not to do it. Example would be a fun game where they tease a dog and the dog whimpers trying to get away. However when the dog turns and growls at the child they no longer consider the game fun because they themselves have been "hurt" or rather scared thus they abandoned the game. Luckily there are kind examples of games where they may play with the dog with both the human child and the dog benefiting each other.
That second example plausibly defeats the statement, but again it takes one action to start toward a path. I do think Defron hit upon an excellent experiment concerning a child though.
Experiment setup:
The BIG RED BUTTON1. In the room is a button on the table and a child "by reasonable definition" is placed in a chair with the button at arms length reach.
2. The child is told that this button decides the fate of the world and that when pressed everyone in the world will disappear forever. This includes their relatives such as their mother and/or father, sister/brother, etc... including their beloved pet.
3. They are then told that if they press the button they will receive a special prize "candy, gum, ice cream, etc..."
Another experiment could be devised to see how cruel a child is by diving another experiment. The button would be linked to a robotic facsimile of a dog/cat and when pressed it would do a dance but would be hurt in the process. There would be no true objective to the game but to let the child press the button as many times as they want. However there would be a sort of "Dark" intent to the game where after a certain number of presses the child receive a negative influence which causes them some discomfort such as a minor jolt like a joy buzzer. They would then be asked if they wanted to continue the game and the responses could then be gauged.
The 2nd experiment might be a better example proving either the statement to be true or only semi-true.