There's no mistake. I agree with Beni. If the test was about spelling, then the mistake was that you need a capital "W" in "Who....?"

This question is perfectly fine. What is it that you don't like about it?

The sentence is grammatically correct. You say you found the sentence in a textbook and with the instruction "spot the mistake". I have had an idea: Did the textbook have any illustrations? Perhaps the sentence is supposed to be describing a specific illustration in your book. For example, perhaps there is a drawing of some girls (not boys) or perhaps there is a drawing of just one boy (not multiple boys), so perhaps the sentence is not semantically correct for the particular context in your texbook. However, it is grammatically correct.

@Beni @Michael @Elisabeth Thank you all, for your constructive and generous responses. I think it is correct after all .. and that it has been some sort of a trick question ! Pedagogicly speaking i think these kind of questions are supposed to make the student criticise the content he is being taught and not just be a "spongy" learner :)