In other conversations in DL Hebrew, the native speakers (moderators) have told us that היא, הוא, זה are sometimes put in nominal sentences and that specifically they are needed when there are two nouns. In this sentence, עוגה and ילדים are nouns and so היא (fem) is necessary. Also, the use of the copula brings definiteness to the sentence apparently: only those kids get the cake, not kids in general (indefinite). We don't need the copula in התפוח הירוק חמוץ, "the green apple is sour," because חמוץ is an adj. The copula is also omitted with people's names and personal pronouns: שרה תלמידה and את תלמידה. There are also tendencies for the use of הוא or זה when there are two nouns, but enough for now.