When I explained unicellular organisms to my six-year-old son Thomas, he asked whether there are any organisms which only have two cells.

Or, if not, the obvious follow-up is: which multicellular organism has the fewest cells?

There are no two-cell organisms I know of (other than during development). I think the answer to your follow-up are the worms eg c-elegans.

There are also some smaller microorganisms like volvox (http://en.wikipedia.org/wiki/Volvox) that blur the line between single-cell and multi-cell organisms.

I think c.elegans has about 1,000 cells. There are some parasites (? rhombozoans) that live in marine life and are composed of 10-50 cells.