Learn Turkish: O çocuklar kim? - Who are those children?

When a pointing word like o (that/those) or bu (this/these) is used right before a noun to describe it, it never takes a plural suffix in Turkish. So, o çocuklar means 'those children', and saying onlar çocuklar would be grammatically incorrect here. You only use onlar when it stands alone as a pronoun (meaning 'they').

How do we know to use -lar instead of -ler for çocuk?

We look at the last vowel in the base word çocuk. The last vowel is u, which is a 'back vowel' (a group of vowels pronounced in the back of the mouth: a, ı, o, u). According to the rules of 2-way vowel harmony, words whose last vowel is a back vowel always take the -lar ending.

Why doesn't kim (who) have a plural suffix to match the children?

You can actually say O çocuklar kimler?, but it's not strictly required. Since çocuklar already has the plural suffix, we know we are talking about more than one person. Turkish often leaves off extra plural endings if the plural meaning is already obvious from the rest of the sentence.

Where is the verb 'are' in this sentence?