With the meals, you typically would use the more appropriate verb 食べる if you are talking about the action - する works when the verb is more like "do", or when the action is "embedded" in the noun (like 話をしましょう)
So, in this case, it sounds like the people might be in the middle of working or doing something, and they say "well, it's about time to have lunch", as in suggesting "let's make the decision to eat" (although we'd never translate it so literally).
It's very common with this phrase 終わりにしよう "Let's finish/end things".
