While I was looking for a job in the Northeast County Times, I saw your ad for a software engineer; which is the job I am looking for.
If the sentence is <em>They enjoyed a snack and then finished their homework </em>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 <em>then,</em> which is also right.What I think could make it better would be to use the personal pronoun <em>they</em> again as in: <em>They enjoyed a snack and then they finished their homework</em>
Vivid nouns put a picture in your head, and non-vivid nouns don't put pictures in your head.