Questions & Answers about Barnen lägger sig sent på helgen.
In Swedish, -en at the end of a noun often marks the definite form (“the …”).
- barn = child / children (indefinite)
- barnet = the child (definite singular)
- barnen = the children (definite plural)
So barnen literally means “the children.”
If you only said Barn lägger sig sent på helgen, it would sound odd or incomplete, a bit like saying “Children go to bed late at the weekend” without specifying which children. In normal speech, if you mean some specific, known children (for example, in your family), you’d use barnen.