My youngest son had this problem. The doctor kept blowing me off, saying it was just a stage, this wasn't unusual, etc. I finally got cranky and insistent, and the doc relented and did an xray which showed my poor lil guy had about 6 pounds of extra stuff that shouldn't be there - he was chronically constipated. This will cause a loss of sensation, which means they can't feel when they need to eliminate, and then have lots of accidents.
I'm hoping that since your daughter has taken the boy to see a physician, this has been ruled out, but as I said, it took THREE visits to my pediatrician to find out the cause of my son's problem.