I read this short story in a collection of short stories about monsters of various kinds. It's told in a journalistic style, with reports about what survivors remembered about a particular kid in the orphanage, with most people remembering him as a normal, unmemorable kid. As a treat, the orphans are taken to see a monster movie at the theater (the story appears to take place in the 1950s); we learn that the best theory of what happened is that the kid was an alien shape-shifter, who was in unconscious disguise as a human, but the movie gave him the idea for a much more interesting shape to assume, resulting in disaster.
Science Fiction short story - orphan child is really shape-shifting alien (monster movie in climax)
This sounds like the short story "Talent", by Robert Bloch, written in 1960. I read it in the anthology "The Baen Big Book of Monsters". The plot concerns the foundling child Andrew Benson, who never learned to talk until the age of seven, but was an intelligent boy who became adept at pantomime. Andrew could also impersonate any person that he saw. After his first trip to the movie house, the boy started speaking and performed an impeccable impression of Groucho Marx, even managing to look like Groucho. Later, Andrew watched a giant monster movie, accompanied by his guardian, Max:
"Max watched him grow. He watched him put forth the eyes, the stalks, the writhing tentacles. He watched him twist and tower, filling the room and then overflowing until the flimsy stucco walls collapsed and there was nothing but the green, gigantic horror." I'd like to see that film, myself.
1That's it! I read it in Bug Eyed Monsters isfdb.org/cgi-bin/pl.cgi?5704 - I was checking Asimov/Greenberg anthologies, but this one is a Pronzini/Malzberg anthology.– AndrewOct 29, 2022 at 21:18
1Good work identifying the story, though I'm not sure what's the point of posting an image of one of the dozen or so collections it's appeared in and not linking to the ISFDB page for the story which lists ALL of them. By the way, if anybody wants to read it, it's freely available from the Internet Archive. Oct 29, 2022 at 21:49
1@user14111 It that case edit the answer to add the ISFDB link. Answers here are a collaborative effort– mmmmmmOct 30, 2022 at 11:53
1@mmmmmm And my contributution to the collaborative effort was to suggest to Mr. Webb how he could improve this and future answers. Oct 30, 2022 at 21:21