Which sentence uses verb tense correctly? They enjoyed a snack and then finished their homework.

If the sentence is They enjoyed a snack and then finished their homework the tense is correct in both parts. You are giving two finished actions in a sequence one after the other, which is correct. You join the two parts by using then, which is also right. What I think could make it better would be to use the personal pronoun they again as in: They enjoyed a snack and then they finished their homework