You're right, the term is abused. If we're following the 'rules' here, anyone who has a predominantly Indigo aura can call themselves Indigo, and if they don't, they're not Indigo - isn't that how it goes? That's where the phrase came from.
I do believe that just because a child has so-called ADHD etc etc, does not make them an Indigo child. There is a hell of a lot more to all of those disorders, and it's to do with the way the world has changed - not only are children today bombarded with toxins from the moment they are conceived, but they also have a lot of very confusing information to process - being a child now is a lot more complicated than it used to be, and the same goes for being a parent. I think a lot has to change about the whole educational system for ALL children, and that has nothing to do with anyone being Indigo or otherwise.
Indigo support forum here