Ben couldn't smell the marshmallows because...
~Maybe Ben's nose wasn't young anymore so it couldn't smell as well as Lindy's.
~Lindy had a better imagination and the marshmallows weren't reallly there she could just imagine them.
~Ben could have had a stuffy nose or a cold.
He can't smell it because it wasn't there!
He couldn’t smell the marshmallows because there wasn’t one. I can’t smell them either.
He wasn't imagining. The marshmallows weren't real. There were no marshmallows. Since he is the oldest, he had a harder time imagining them being there.