I might approach it this way.
" I think that under the circumstances the photos came out just fine. Sometimes people have difficulty having their photo taken. There are steps that can be taken that will make the photos turn out better. Clothing choice, lighting, location, attitude.
If you would like to try the session again I'd be happy to re-shoot it. It would be best if we agree to the details before hand so the session will be go as smoothly as possible."
And, if you don't want me to re-shoot I will completely understand that you are just social idiots who couldn't get a good photo taken if you paid a million dollars.

did I say that outloud ? My Bad...