A lot of times kids will have some idea of what sex is, but they will also be completely wrong in bizarre ways. They sort of know that it involves genitals, but sometimes they think it involves disconnecting the genitals and throwing them, or the man giving the lady his junk to keep, etc. They kind of get the abstract, but the specifics evade them.
When I say that this is a movie that feels like it was written by a child, I want you to understand that I am not insulting the quality of the writing, but rather, pinpointing something weird about this movie’s world building. When E.T. gets drunk he gets drunk like a child thinks adults get drunk: he opens one Coors light, then gets cross eyed and starts stumbling. When Elliott had to dissect a frog for science class it’s exactly as nightmarish as a kid would imagine dissection to be – “here’s a live animal, now kill it!” - rather than the much more sensible “here’s something dead, now open it up and take notes” that actually happens. The Halloween scene is exactly what a kid wants Halloween to be – there’s a riot’s worth of kids toilet papering every house, and there seem to be open barrels of fire getting rolled down the street – but I doubt Halloween has ever looked like that in America. When a government agent shows up your front door to steal your alien from you he’s dressed in an astronaut suit, because of course the agency in charge of aliens is NASA, and that’s what NASA people wear, even when they are driving to suburban houses.
Because this movie has that childlike viewpoint, it ended up speaking to kids like almost no other movie had ever spoken to kids before. Which is great for kids, and great for the people who collected the massive box office receipts. But that viewpoint is a bit of a problem for adults, because to someone who has gotten drunk this movie feels weird and often inauthentic more than it feels imaginative.
Which is not to say that an adult can’t appreciate it at all; it’s still got a sweet heart, and it’s often pretty funny. But it is to say that as a kid you totally miss how weird it is that they live in a southern California subdivision and yet magically live in front of a gigantic forest, or how it’s weird that their shed is missing a wall and is lit up twenty four hours a day by baseball stadium lights, but that sort of stuff is all you see when you watch this as an adult.