"Who is eating lunch under the big trees?"
Translation:Kik ebédelnek a nagy fák alatt?
Wouldn't the singular also be correct, since the sentence doesn't indicate a plural who in any way?
Yes, the singular should be accepted here. There's no way to tell which it is from the English sentence.
It's possible. There are really only three parts to the sentence, though: the interrogative work kik, the verb ebédelnek, and the phrase a nagy fák alatt (which can't be broken apart or rearranged in any way). The interrogative needs to come immediately before the verb. So if you want the verb at the end of the sentence it can be done only one way:
A nagy fák alatt kik ebédelnek?
Thank you, jsiehler, so I can conclude that subject and verb must come together in an interrogative sentence (?)