Looking over the various discussions of these and other similar quotes, leads to the conclusion that Einstein probably did not say any of these, exactly. I have found no direct attribution to Einstein of any of these quotes of 'six-year olds','barmaids' or 'grandmas' is mentioned. Of course you cant prove something doesn't exist.
The closest, earliest article I find that these modern quotes seem to be based on is in the 1962 book New Perspectives in Physics, where the author states that Einstein, when discussing theories,
...ought to lend themselves to so simple a description "that even a
child could understand..."
This seems the closest to most of the modern 'quotes'
That these quotes are all paraphrasing Einstein I do not,however, doubt. A couple of earlier sources show this attitude towards trying to express his theories, and science in general, in a simple manner.
---From the 1938 book,The Evolution of Physics , published by Leopold Infeld, Albert Einstein :
Most of the fundamental ideas of science are essentially simple, and
may, as a rule, be expressed in a language comprehensible to
---Another, earlier yet quote shows Einsteins concern with simplicity in science and his theory
... Its great value lies in the logical simplicity with which it
explains apparently conflicting facts in the operation of natural law
It provides a more simple method. Hitherto science has been burdened by
many general assumptions of a complicated nature.
The above was published April 8, 1921 in an interview labeled "Prof. Einstein--The Poet in Science"
So, unless someone finds an actual, published quotation, my conclusion would be that these quotes are all modern paraphrasing of things Einstein did, at various times state. They just aren't his words.