Very nice demo! Quite a lot of game compared to what demos normally give these days.
I love the game's environmental storytelling! A small, connected community in Sweden(?), the world from the Stargate movie - which I appreciate the kids having no idea about given the film first released in 1994, 5 years after the game starts - looking to have been studied by humans, only to have been attacked, the reality-linking apartment complex which feels straight out of Control, and the torn-apart town after the apocalypse.
The writing is also quite good. Nails the feeling of kids grown up on material far above their age range.
My main complaint is a lack of polish. Houses you've already trick-or-treat'd at don't let you interact on their doors again so the player can be reminded they've already been there, the abandoned military base well off the beaten path in the first area doesn't have the characters comment on it, areas feel emptier than they should (i.e. the ruined town should have more things to look at, read, etc.), the kids have no comments on repeated dialogue once they return from Stargate World (i.e. after the big kids inviting the kids to see their fireworks, there's no confused conversation where the kids think the fireworks have already been set up), and so on. Just a lot of little things like that which add up.
There's also some areas which feel big enough, but would really benefit from the kids being able to run - especially when backtracking or searching for the next thing.
As for hinting at the safe puzzle's solution, there could be a note, a diary entry, something which gives the perfect, succinct hint you gave to another player: the safe's combination is something stupid. And if the player keeps guessing the combination incorrectly, that can trigger a conversation where one of the kids suggests trying the actual solution, with the others being incredulous because what adult would use such an obvious password?
All said, for what is probably an alpha, this was fantastic. I can't wait to see where this goes next!