Final answer:
The correct verb form is 'les interesan' and it is used when referring to a plural subject.
Step-by-step explanation:
The correct verb form to fill in the sentence is les interesan.
In Spanish, the verb form changes depending on the subject and number of the noun. In this case, since 'los niños' refers to a plural subject, we use the third person plural form of the verb 'interesar', which is 'interesan'.
Example: A los niños les interesan los dibujos animados.
Learn more about Spanish verb conjugation