Post an adult-like child oder childlike adult.

Examples:

Manchild: Soma from Kuroshitsuji. Hyper, goofy, impudent, childish, doesn't behave properly in public, limited sense of self-control, naive. The woman he was looking for was not his sweetheart like the Anime suggests but his substitute mother figure.

Creepy kid: Kaiba Mokuba from the Yugioh Manga. Notorious from bribing & cheating. A gradeschooler with anger management problems & a penchant for trying to kill people. For example, he once threatened Yuugi with a messer & latter poisoned Jounouchi. He made it very clear that he wanted them both dead.