The older kids in Stranger Things were, for the most part, playing a different game, as you said, but their game still overlapped with the younger kids' game, if that makes sense.I'd be thinking that Eleven and the Three Boys is a basic gaming group without having to worry about the older kids; they're playing a different game.Barb was barely an older person, being part of the group that three of the older main characters came from. Or are we just focusing on Eleven and the boys?
I don't think the point is to kill the Kids, but the real danger of their actions should be considered and made aware to the Kids, if not by game mechanics, then possibly storytelling. This might be more of a homebrew thing, though.
The "Can't be killed" could be abused, I suppose. So as a GM, I would be inclined to have them seem as if they're killed like in the old Cliffhanger serials but they can re-appear later. IF I feel the player has abused the privilege.
As for the "can't be killed" aspect being abused. I'm totally going to homebrew in some hit points for the Kids and whatever they come up against. For the most part, the game is very non-combative, and that's great, but sometimes knowing how much life someone or something has left is also extremely helpful to further the story. Plus, it adds to the risk management when the players attempt something that might be dangerous to their Kids.