Most of the reviews that say this is an intelligent movie and "you have to be smart enough to get it" are written by less intelligent people (sorry) who don't usually "get" films and are excited to have understood this one - because it is spelt out for you. There is nothing deep to be found here. I might have found this "deep" and "intelligent" when I was 12, probably, but probably not even then. Any deeper meaning is distinctly and clearly spelt out for you - "You're stuck in the past, you work at a museum!" - as is every. single. thing. that happens.
I don't believe the writer has ever had an interaction with another human. No one, not a single solitary person, talks like this. The characters incessantly give a running commentary on what's going on, the characters are literally introduced by children asking "your name and occupation?", and sentences feel as if they're generated by an AI impersonating a writer. It was so bad that my partner and I had a running commentary on how random and forced the lines were. I do think the actors did the best with their awful script.
Plotholes are huge and whether or not the children have mentally aged fluctuates to suit the script (and they gain a whoooole lot of knowledge out of thin air and from experiences in the world that they never actually had, like prom and how they've missed out on it) - 6 year olds having sex? It's okay, their brains aged! Oh wait, they're making sandcastles at 50 and still talk like children.
Concept was interesting enough to give it one watch and never again.