shouldn't go to sleep be also correct?
They may be going to bed early, but to sleep much later.
Yngre søsknene? Hva betyr det?
Nothing. I think you meant "søsknene".
yes, sure....but "younger siblings"? Younger by few minutes I suppose, or not?
Not in this context. Just younger brothers and sisters. Little sister and little brother.
i never picked up on that spelling before...
Why " the younger siblings of mine go to bed early" is not accepted?
Is the 'de' really required?
I have a hunch that it must be so
Yes it is.
why not lie down early ?
Because this isn't "lie", it's "lay", which is why it requires the "seg". "They are lying in bed early" vs "They are laying themselves in bed early".
Because you can lie down on the floor, the couch, wherever. But you can only "go to bed" ("legge deg"), as in put on your PJs and sleep for 8+ hours in a specific manner.