There have been numerous papers written advocating that twice daily administration of hydrocortisone is insufficient for anyone with CAH as it only lasts in the body for 6-8 hours (and that’s ’normal’ metabolizers!).  I think there are very few endocrinologists who still have their patients on a twice daily regimen as studies have proved that hydrocortisone is required more often - some kids even have it 4 times a day.  My son took his hydrocortisone twice daily until the age of 8 years (though changed to thrice daily after taking part in one of the said studies).  His general well being most definately improved as did his blood test results!  I cannot understand why your childrens endocrinologist is reluctant to try this method to see if it brings their CAH control back in line. 

Personally I would be  wary of changing to prednisone, due to it’s higher potency and the possible detrimental effect on final height (if the dose is not finely tuned, which is quite difficult on this medication).