@selectmytutor,
Thank you. I asked somebody in my office and he said child was right.
The reason is, only can only be followed by singular concept, not singular noun, but concept.
For example;
There are only one pen left on the table.
there are only two pens left on the table.
here, "two pens" is a unit concept, a singular concept, pens follow two, not "only". "two pens" follow "only".
It would be wrong if you say "there are only pens left on the table."
Certainly, if you want to emphasize there are only pens, no pencils or rulers whatsoever left on the table, the sentence is correct again. "pens" is a unit, singular concept, not a plural concept.
Please correct me if I am wrong.
Thank you very much for your comments!