Breakdown of De stränga föräldrarna låter inte sin son slippa sysslorna.
inte
not
sonen
the son
föräldern
the parent
sin
his/her/its/their own (en-word)
Questions & Answers about De stränga föräldrarna låter inte sin son slippa sysslorna.
Why is there no att before slippa?
The verb låta (to let/allow) acts as a causative verb here. Just like in English where you say "let him skip" (not "let him to skip"), Swedish causative verbs like låta and få take an object (sin son) followed directly by a bare infinitive—a verb without the infinitive marker att.
Since there are two parents, shouldn't it be sina son or deras son?
Swedish uses reflexive possessive pronouns (sin, sitt, sina) when the object belongs to the third-person subject of the clause. Since the parents are dealing with their own son, we avoid deras (which would mean someone else's son). Even though there are multiple parents, the form of the pronoun depends purely on the noun being owned. Because son is a singular en-word, we use sin.
Why do we need De, stränga, and -arna all at once in De stränga föräldrarna?
This is a great example of Swedish "double definiteness." When you want to use an adjective with a definite noun, you have to sandwich the adjective between two definite markers: the standalone plural article (De) and the definite noun ending (-arna on föräldrar). The adjective itself also takes its definite/plural form (stränga).