We loved GG and went there many times, but due to the multitude of absolutely lousey reviews since the "refurb", we won't be going back.
CM is very loud. My son enjoys it. He is 7. My daughter was overwhelmed and kind of scared when she was really small. She enjoys it now that she's been a couple of times though.
Have you thought about 1900 Park Fare? The food is a step above the other buffets, and the characters may be more young-kid friendly. For breakfast it's Pooh and friends, Mary Poppins and friends, Alice and friends. We've also met Pinocchio there. Dinner will have you meeting Cinderella and friends. More face characters at this meal which is normally easier for the young children.
Of course, if you are set on Mickey, (which is of course always a good choice!), CM is the better of your two choices. Don't forget about Donald's Breakfastosaurus at the AK. Mickey, Donald, Pluto and Goofy hang out there too. They have a really "meaty" buffet there for breakfast. They also have a lot of fruits, yogurt, oatmeal, waffles, etc, but with my carb-hating hubby, we found this buffet to his liking.
A quick glance of all character meals and who appears at each one can be found on the Allears site:
http://www.allearsnet.com/din/chglance.htm