Why not "They are eating"? "eating" is one of the solutions, according to the drop-down menu
If we you wanna use continuous tense, you should put word "sedang" before the verb.
They are eating can be both translated into:
"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