Learn Icelandic: Barnið fær tvö rúmstykki og eitt kex. - The child gets two bread rolls and one biscuit.

For neuter words, the numbers 1 to 4 are identical in both the nominative and accusative cases! The verb fær (gets) does make its objects accusative, but because rúmstykki (bread roll) and kex (biscuit) are both neuter, you use tvö and eitt whether they are the subject or the object.

Is rúmstykki plural here? It does not look like it changed from its singular form.

Yes, it means 'rolls' here! In Icelandic, many neuter nouns look exactly the same in the singular and the plural. We know it is plural in this sentence because the number tvö (two) is right in front of it.

I thought the verb for 'to get' was fá. Why is it fær here?

Fá is the basic dictionary form. Verbs change their endings based on who is doing the action. Since the subject is barnið (the child, which acts as a third-person 'it'), the verb changes to fær. This is an irregular verb, so its vowel also changes from á to æ.

Why does the sentence start with Barnið instead of just Barn?