Learn Icelandic: Jú, hann svarar þremur börnum. - Yes, he answers three children.

The verb svara (to answer) always forces the person or thing being answered into the dative case. Because the Icelandic numbers 1 through 4 change their form to match the noun they describe, the number three becomes þremur to agree with the dative noun börnum (children).

Is þremur specifically the neuter form to match the neuter noun börnum?

No, þremur is actually the dative form for all three genders! Whether you are talking about three boys (masculine), three girls (feminine), or three children (neuter), you will always use þremur when the phrase is in the dative case.

Why does the sentence start with jú for "yes" instead of já?

Jú is a special kind of "yes" used specifically to contradict a negative question or statement. This sentence is likely replying to someone asking "Doesn't he answer the children?" By using jú, you are clarifying "Yes, contrary to what you just suggested, he actually does."