In a distant ruin, Morgaine Le Fay and her young son Mordred arrive. Mordred is impatient for something to change, but Morgaine assures him that she wants him to be young when she gives him his kingdom. They come to a statue holding the Amulet of First Magic. Morgaine fires a bolt of mystic energy to retrieve the Amulet. The statue's hand open and Mordred takes it. When Morgaine asks for it, he wonders why he should, dons the amulet, and tells her that he's strong enough to defeat her. He covers himself in mystic armor and casually deflects her mystic attack. He then informs her he doesn't wish to see anyone older than him ever again. Mordred casts a spell that radiates out throughout the world. Morgaine disappears in the first wave of the energy. In an elementary school, the teacher disappears and the children cheer. At a zoo, a nagging mother disappears. A policeman disappears before he can arrest two young taggers.
Superman, Green Lantern, Wonder Woman, and Batman are taking on a team of villains: Copperhead, KGBeast, Cheetah, and Blockbuster. The heroes prove triumphant but the mystic waves sweeps over them despite Green Lantern's efforts. Heroes and villain alike disappear, only to find themselves floating in limbo with all the other adults on the planet. Morgaine appears to explain that Mordred is responsible and has banished all adults to the shadow dimension. She's unable to overcome the power of the Amulet and suggests they work together. Morgaine notes that the spell only banishes adults and the heroes have no choice but to agree. Morgaine casts her spell and the heroes disappear from the shadow dimension.
Mordred arrives at an amusement park to rule over his new subjects. He transforms the park into a twisted medieval setting and the children follow him for lack of anything better to do. They're unaware that four other children are behind: kid versions of Batman, Superman, Wonder Woman, and Green Lantern.
The heroes start to come to grips with their new bodies and Kid Green Lantern creates a pair of glasses. They go into Mordred's new castle, the flying Leaguers outracing a frustrated Kid Batman. Meanwhile, a bored Mordred tries to satisfy his new subjects. One girl has a hungry baby sister but Mordred can only summon a cow for her. The heroes arrive and Mordred animates toys and transforms them into giant warriors to battle them. Kid Batman manages to temporarily stun Mordred and then trick him into firing a mystic blast and destroying one of his own automatons. Kid Green Lantern starts to get the hang of his ring and the heroes defeat their mindless opponents. They go for Mordred, who casually encases them in crystal and then banishes them to the dungeons below
As the heroes look on in horror, Mordred releases a dire beast from its cell: the Demon Etrigan, similarly transformed into a child. They try to restrain him without harming him. Finally Kid Wonder Woman applies some discipline and then comforts the baby creature. When the baby makes a mess, they hand it off to Kid Superman and make a hasty exit.
On the surface, Mordred is letting the kids run wild. The heroes finally take command and tell them to wait outside. The Leaguers then sneak back into the castle where they find Mordred sleeping on his throne. They split up and Kid Green Lantern tries to remove the Amulet. One of the children, a young girl named May, comes back, waking up Mordred. The Kid Leaguers jump on him but Mordred grows to giant size and throws them off. He summons stone dragons and sends them after the flying heroes. Kid Wonder Woman defeats hers while Kid Green Lantern escapes with Kid Batman on a flying disc. He maneuvers their pursuer into a bridge, destroying it. Mordred follows them back toward the castle but they seal the doors to keep him out.
Kid Batman has Kid Green Lantern go crazy and the hero creates a giant green battlesuit. Mordred summons his own suit of armor and goes into battle, but realizes that it's a distraction for Kid Superman to try and take the Amulet. Kid Superman gets the Amulet and throws it to Kid Batman, who throws it to Kid Wonder Girl. Mordred mystically ensnares her, but she throws it to Baby Etrigan, who destroys it and ends the spell.
Mordred gets back to his feet, informing them he's already absorbed much of the Amulet's power. He prepares to kill them but May comes back and asks for her mother. When she starts crying, Mordred is frozen with indecision. The heroes taunt him, pointing out he could have used magic to make himself a man at any time. Mordred takes their dare and casts a spell to become an adult. Once he does, he disappears and appears in the shadow dimension. Morgaine is waiting for him and explains that he's used all of his power. She warns that it's too late for him now.
On Earth, the spell fades away and all the adults are restored to the planet. Morgaine appears to the Leaguers and restores them to their adult forms. As she goes, Batman asks what happened to Mordred. Morgaine says that with the spell broken, he now has eternal life without eternal youth. She teleports away and Batman looks at the happy children reunited with their parents. Wonder Woman notes it was enjoyable, but Batman admits he hasn't been a child since he was eight years old.
In a distant castle, Morgaine tends to her decrepit son.