Mom,

I have read about fecal retention in youths who have a painful dry stool at some point and associate defecation as a painful experience. The solution most often offered in this situation is something along the lines of giving the child some laxitive and explaining that as long as they drink enough water going to the bathroom shouldn't be painful.

See the section on Functional Fecal Retention here:

http://www.aboutkidsgi.org/FecalRetention.html