It is the same in Belarusian (stress on the first syllable since there is also a word with a stress on the second syllable) so even more opportunities for jokes:)
I'm not sure what the intended meaning of this sentence was but i think "oni" should be omitted or placed first. If someone wants to say what they will be doing soon should say: "(Oni) będą zaraz jedli" or "(Oni) zaraz będą jedli" (będą zaraz/zaraz będą - the order doesn't matter). But the sentence from the exercise is the answer to the question "who will be eating soon". Than yes, "oni" is needed and if one wants to put more emphasis that it's about them (not about us or you), he can place "oni" at the end - "Zaraz będą jedli oni".