This kind but firm approach will achieve more than harsh punishment,
which might entail many negative consequences [unintended by the
"Unintended" is not an adjective here, but a past participle verb heading the bracketed past-participial clause modifying "consequences".
Past-participials (and gerund-participials) as modifiers in noun phrase structure are semantically similar to relative clauses: cf. "negative consequences which are unintended by the parents", but we don't analyse them as relative clauses since there is no possibility of them containing a relative phrase (cf. *"consequences which unintended by the parents").
Past-participial modifiers are 'bare' passives, as evident from the admissibility of a by phrase.