My mom remembers reading a science fiction short story as a kid... It's about some alien invaders who are gathering reconnaissance and end up reading a couple of kids' comic books (possibly in their treehouse?). The aliens think the comic books are the boys' journals and think Earth is defended by creatures with amazing powers (the superheroes) and decide not to invade.
Summary from the Recursive Science Fiction site:
Earth is about to be invaded and humanity destroyed. A scout from the invasion fleet steals [the] contents of a tree house and returns for evaluation. Some of the items are science fiction comic books. The invaders, having no fiction in their culture, take them to be historical records and flee in terror.
Excerpt from the story:
He discovered the Council was waiting for him. Frowning to cover his embarrassment, he took out the last of the Scout’s finds. For a moment all of them were struck by the bright colors on the flat surface. The one old man reached out a trembling hand. “Records,” he murmured incredulously. “Records such as our own race is said to have once made, long long ago before Law.” Reverently, he examined the cover, then with remarkable agility for one so decrepit he jumped to his feet and flung the thing from him. His face twitched with horror.
The others shocked and disbelieving, fell to examining the rest of the new articles. In a moment, cries of alarm filled the council room. Chairs were upset, dignity forgotten. Only the eldest retained his composure, although with difficulty, for he could hardly manage to control the palsied shaking of his hands. The astonished Commander leaned over his shoulder and watched as the ancient turned the pages.
What he saw made the blood drum in his ears, made his vision swim, and only faintly did he hear the old one’s croaking words. “Praise to Law, whfch we so carelessly accepted, for Law has saved us from the fiendish denizens of this planet. Had we attempted to exterminate them, their space armadas would have taken instant revenge. For they are obviously mightier than we.” He put down the bright record of space craft vaster than the one which they occupied and took up another. On its cover was depicted a world being blasted into flaming wreckage, and within was shown the pictorial history of a space fleet, engaged in repelling an alien invasion, and who followed up their successful repulse by annihilating the entire system of the aliens.
Five more of the record books did they examine before the Commander’s stunned mind at last reeled beneath the hideous concepts and he could look no more. Dumbly, he managed to reach the phones and order the ship thrown into emergency drive to some far and lost point in space and dimension.
And as he waited for the shuddering wrench that signalled interdimensional shift, he tried to forget the horrors they had so narrowly escaped: Creatures who could make themselves invisible, who had mastered space travel, who worked in magic more powerful than that of Law’s, who could whiff out entire solar systems, who could survive incredible mishaps and hardships. Creatures who were no less than Gods!
Another short story on the same theme is "Reconnaissance" by P. W. Cutler in New Worlds Science Fiction #30, December 1954, also available at the Internet Archive; but this one is less likely to be the story your mom remembers, because it does not mention a tree house, and because it seems not to have been reprinted. Summary from the Recursive Science Fiction site:
"Alien invaders are accidentally bamboozled by humans, in this case a kid who shows them his comic books and his older brother's copies of New Worlds and The Conquest of Space so they flee in terror of this obviously superior civilization (serving them some of his father's whiskey helped too). P(eter) W. Cutler is, I believe, a fan very briefly turned pro (total output, this story and one in Science Fantasy the same year)—at least his name is familiar from the letters column of earlier issues of New Worlds. This story is notable for an early appearance outside fanzines of the term "zap gun," applied to the protagonist's de luxe water pistol." John Boston by permission via Dennis Lien