Question: How do I tell my child he/she is on the autism spectrum?
Answer: The key is to be open to the nature of the child's difficulties, but to do so in a way that is appropriate for their developmental level.
It's also important to frame difficulties as just that -- that using their words, or dealing with friendships is difficult for them, but not impossible.
You wouldn't want to use the language of a deficit that says that you're unable to do something because you have an autistic disorder.
As children get older, it's appropriate to use words like autistic disorder, or autism -- but to still do so in a very simple way with very specific explanations about those things that are difficult for that child.
When children are teenagers or older, and if they're developmentally able to do so, it's very appropriate to use other things like sources of reading, or books, or more background information on the nature of the disorder.
They should do so, though, in a way that respects the child's willingness to take on that information. Not force them to read about it.
For very, very young children, I would encourage parents to not use terms like autism or Asperger's disorder or other technical terms, as kids simply might not be able to use those.
And to especially emphasize simple descriptions of those things that are hard for that child to do, with an assumption that they need to practice at it, and they can get better.