I feel like the french cartoon, Spartakus and the Sun Beneath the Sea/Les Mondes Engloutis should count as a softer, more child-friendly take on backrooms/liminal space type horrors. Two kids accidentally get lost and are transported away from the real world. It is never explained how or why. The history of the worlds they see in the stratas does not seem to align with the history they know (they do not seem to know about a big catastrophe that shook the world) - but the places they travel through still seem to be twisted mirrors of their own reality, with fiction sometimes meeting history. There are places which look like outdoors locations, but mostly there is ceiling, and they are still in caverns. We see later that it is possible to get trapped in other compact spaces, simply by touching objects. There are some weird and empty spaces they travel through. There are entities that show up at several places, ghostlike creatures, monsters, temporal distortions, and it is never completely clear, if the kids truly got back to the real world, or just reached a very similar space. And yes, we see relatively few truly scary things, but we should also note, that one of the main characters (Spartakus) travelled these spaces for more than ten years. He had the time to gain a good understanding of the layout, the creatures and dangers there, so he can effectively avoid most confrontations, and protect the team.
