What should special needs children be told about their disability?

Depending on their level of functionality, explain as much as you can. These children need to learn coping skills. The first step to that is awareness. If they can self-regulate their symptoms, then they can self-motivate the solutions. For example, when my son has hyperness or focus issues, he needs to recognize it, then recall his behavior mod lessons and choose to either go off to a quiet spot or run outside to get his energy out, etc. He is high functioning autistic so he had to learn that spinning makes him more wound up so he needs to choose an alternative behavior for getting out this sensory craving. Just as an allergy child needs to know what will 'cause a reaction, a special needs child needs to know the obstacles to overcome them.