Learn Turkish: Çocuklar nerede? - Where are the children?

The word nerede literally means 'at what place'. It is built from ne (what), re (place), and -de, which is the location suffix meaning 'at', 'in', or 'on' that you learned recently. When you ask about a location, that location suffix is built right into the question word itself.

Why is there no word for 'are' in this sentence?

In present tense Turkish, the 'to be' verb (am, is, are) is usually invisible, especially for the third person (he/she/it/they). Simply putting the subject (çocuklar) and the question word (nerede) next to each other is perfectly enough to mean 'The children are where?'.

In English, 'where' comes first. Can I ask Nerede çocuklar? instead?

You will sometimes hear people say it that way in casual spoken Turkish, but standard Turkish word order is Subject + Question Word. Because of this, Çocuklar nerede? (literally 'Children where?') is the most natural and grammatically proper way to ask.

Just as a quick refresher, why is the plural suffix -lar here instead of -ler?

This follows the rules of vowel harmony. The last vowel in çocuk (child) is u, which is a 'back vowel'. Back vowels always pair with the -lar plural suffix. If the word had a front vowel like 'e' or 'i', it would use -ler.