I'm going and assume that there is a war with the humanity and demon kind and that war took very long time, because if what was thinking is right then that the parents are being force to leave they're demon children because (most likely) of the war with the humans or (not that likely) a calamity hits the earth and the reason is
1. Lorem and meri are both orphans and almost any other character in the series.
2. The thatered clothing and ruined buildings is also a sign of a long war.
3. in one of the chapter one dude try to recruit Lorem to fight against humans.
4. Demon landmine and weapons against demons are still around even until to this very day of Era where mostly years after the war, signs of this can be seen with the Demon rehabilitation with humanity and co-existance with demons
5. Slavery is a thing with the Demon society, i think there society is a somewhat medieval,(I will explain this in number 6) but we can whole heartily say it's still there or some parts of it.
6. I say there society the demons are somewhat mediaval because they mostly use there power and swrods (as I can overserve) and they can't really fix the buildings the humans made because as we can see when Lorem and the others infiltrated the Demon city almost everything is ruined or just not in good shape
But overall this is all my observations as far as I can say for now at the very least.
I hope that I can help you guys with this little conservation of mine.