Final answer:
Malcolm means that Macduff's noble passion is a product of his good character, inspiring Malcolm to give up his vices and claim the throne.
Step-by-step explanation:
When Malcolm refers to Macduff's "noble passion" as a "child of integrity", he means that Macduff's passionate response is a product of his good character. Macduff's ideals and values are virtuous and noble, which has inspired Malcolm to reassess and abandon his vices. Malcolm recognizes that Macduff's passionate response stems from his integrity and genuinely convinces him to claim the throne.