OP- "Will we be disappointed?"
Well it will never be the same as when your children were 6 & 9, which is where I normally point out that we are all part of making the magic happen; typically more than you think. It's the group you are with, your attitude, expectations and if it is something new it is part of an adventure as you have no expectation. Go to Mickey's from a position its new, it is a brunch, your children are older and you are celebrating time with your family I would think you should be fine. Just remember it is a major destination of very young diners and as others have pointed out it will be louder than other dining options. On the other hand, if you are concerned about being disappointed to a level where you can't drop the thought/concern, I would select another place to break away from pre-judging an experience. Just remember reviews are only opinions, but negative trends may indicate a level of concern if accurate.