If your child is far enough along in school, they've already had "tests" and "assessments." Mine loves tests, so he didn't ask for much explanation.
We did talk about how he has an interesting mind and by doing assessments we can find out more about what he knows and what he still needs to learn. "The people in this office are experts in giving assessments."
If your child is old enough to know that not everyone gets these tests, you can express to him something you think is unusual about him (not something bad, just different) and say that the assessment might help us understand "your special quality of XYZ".
I wouldn't offer suspected diagnostic terms right away-- too confusing to have it change if the diagnosis doesn't fit after all. I'd wait until it's a sure thing, and you've come to terms with it yourself, before teaching the child about it.
Best wishes,
DeeDee
