Jan,
 My nephew like to nurse with his "gahkee" (blanket) in his mouth. My sister
discouraged it because she found it painful. He continued to suck on the
blanket when off the breast, and after weaning at age four he kept his
blanket for sucking on.
As long as the mom doesn't find his thumb uncomfortable and he is still able
to suck effectively, I don't see much of a problem.

Heidi Gleber, LLLL