If we're trying to be more real, we should have more young childish villains. Most of the problems we see everyday are caused by unruly, undisciplined delinquent adolescents or children who have suffered some kind of trauma as abuse or negligence. I'm not saying they should be the big bad because realistically "children" are not capable of being masterminds who plan for every contingency as an arch villain should. But they can be the Big Bad's Dragon, the monster, the experiment, the perfect soldier what-have-you. "Children" saving the world..? Unlikely! "Children" ruining the world... more likely everyday.

Don't get me wrong. I'm not suggesting ridiculously young villains. 12 is the youngest I can imagine a criminal to be considering intent. (SPOILER)At such an age, they should have proper knowledge of right and wrong and understand that lying is worse than killing especially if it was an accident. Usually when a child is the cause of someone's death, the parents are to blame for covering it up, but if it was the child who lied, there would likely only be two reasons. 1) fear of consequence from adults... the child would have to have previous experience with physical punishment over "light indiscretions" to fear punishment from an adult for an accident even if someone died. 2) fear of prosecution... IMO at age 12, a child is old enough to understand that lying about an accidental death would get them into more trouble than being honest. However, if they intentionally hurt someone, they're gonna lie because they can't grasp the legal definition of murder versus the simple knowledge that they caused a life to end.
Make no mistake: you may have heard tell of a younger child explicitly stating they wanted someone to die but I deem only those 12+ to be capable of understanding what that wish truly entails. Also, the ability to prosecute a young person for murder does not apply to a young person's ability to premeditate.

An example of how children could be utilized as "villains" could possibly be an army of 12-year-olds (perhaps clones). Another example could be a small group of young people (maybe they approached the Big Bad for mutual trade such as removing horrible memories in exchange for his requisite villainous deeds).