Litteraly" Who are having lunch under the big trees." Ebédelnek ( ők)- they are having lunch
Is "Kik" plural
if so shouldn't it be translated as "who are..."?
No, because that's not how we usually say it in English - who as a question word (rather than a relative pronoun) takes a plural verb usually only in the connection Who are [plural noun]? but not with other verbs than to be such as eat.