If we you wanna use continuous tense, you should put word "sedang" before the verb.

They are eating can be both translated into:

"Mereka makan"

And

"Mereka sedang makan"

The word "sedang" is actually optional. Remember, Indonesian is a flexible language with no tenses. To describe past, future and continuous tenses, we just add descriptive words without changing the base words and or add grammatical rules.

I ate - Saya makan

I was eating - Saya makan

I eat - Saya makan

I am eating -Saya (sedang) makan

I will eat - Saya akan makan

I will be eating - Saya akan makan

Etc.