From what I've read Cushings is basically a hormonal imbalance of some kind. It causes excessive sweating, they usually don't shed out their winter coats easily, they can get fairly chubby, and can't be worked as hard.
Between that and the founder you need to make sure she is on a low sugar low starch diet. No fresh green grass in spring and early fall...no sweet feed...no corn. I HIGHLY reccomend Triple Crown Low Starch feed. It is fantastic! There is also a new Purina WellSolve Low Starch...but I haven't tried that and I've heard it is super expensive. Stay away from Safe Choice and Acco Safe and Sound...they are gimmicks...and are only low fat...NOT low starch/sugar (no matter what the feed salesman tries to tell you). You need to make sure that the feed has a low NSC %...non-structured carbohydrate.
Cushings also increases the risk for osteoarthritis so you may want to think about a basic joint supplement.
A good friend of mine has a horse with Cushings...his treatment includes some sort of homeopathic drops that are just put on his food with an eye-dropper every morning...not sure what it is though? But it was much cheaper and safer than the Peroglide stuff that is usually prescribed.
Cushings horses also can't be stressed out to much or they will have a cortisol induced stress reaction (because their hormones are all mixed up anyway). So I would just make sure that your kids understand that...and tell them that screaming and/or the always fun racing friends on horseback is probably out of the question on this particular horse.
Other than that you just need to make sure she stays groomed because the excessive sweating will cause salt build up.
That friend of mine I mentioned...her horse was diagnosed with Cushings last year and now he is back in light riding and even doing some halter classes at small local shows with her husband this year.
So don't get to depressed...there is always hope...just problems like this are labor intensive.