The symptoms of, especially, autism vary widely.
One child with autism could present severely. He/she is constantly on the move, stimming, unresponsive to any person. Then, when the child has stopped moving, he/she might sit in a corner and rock back and forth, and perhaps abuse himself, thereby requiring the constant wearing of a helmet.
Another child could walk and talk and do things to the point where he/she is superficially indistinguishable from "normal" people.
People with ADHD don't have as much variance in their symptoms as people with autism.
One child with ADHD might be "on the go" constantly, cannot sit still, has a short attention span, and has trouble responding to people. Usually, even amongst those severely affected, there is no cognitive impairment.
Another child might merely present as being apathetic.