Learn Turkish: Çocuklara her gün vitamin vermek gerek. - It is necessary to give vitamins to children every day.

It uses the infinitive verb vermek (to give) immediately followed by the word gerek (necessary). Together, vermek gerek means "It is necessary to give" or "One needs to give". You could also swap gerek for lazım here without changing the meaning.

Why does çocuklar (children) have an -a at the end to make çocuklara?

Because of the verb vermek (to give). In Turkish, when you give something to someone, that recipient needs the "to/towards" ending (called the dative case). So çocuklar becomes çocuklara (to the children).

The English translation says "vitamins", so why isn't it plural (vitaminler) in Turkish?

When talking about a general category of things rather than specific items, Turkish often uses the singular form without any endings. Vitamin vermek just means the general action of "vitamin-giving".

Why does gerek come at the very end of the sentence?

In Turkish, the core of the sentence's meaning usually goes at the end. Even though gerek is a word meaning "necessary", it acts as the main statement here, so it takes the final position.