I take serious issue with this terminology. First and foremost, because nobody died. Despite what mainstream media and organizations may say to the contrary, your child is still alive and (most likely) thriving, just in a different way than you expected. Equating disability to death and using the exact same scientific grief scale implies that those two are one and the same, equally horrible outcomes and that people should and will react to a disability diagnosis the same way they will react to a death.
To be clear, I'm not talking about parents whose children suddenly acquire a disability. That's a whole other can of worms. It's different in that situation, because there is a normal child to grieve. And though it's true that no one died in the traditional sense, that child will never be the same. Their nondisabled self, for all intents and purposes, is dead. And it doesn't make their life any less worth living, but it does radically change things. They already had an established life, a nondisabled life, and now they have a completely different life, a new life. So in that circumstance, I think it's acceptable to grieve. But for disabilities that are present from birth or very shortly after, like Down Syndrome, or cerebral palsy, or autism, there was never a "normal child". This is your child's default setting. This is the way your child always has been and always will be. Grieving for a "normal child" implies that there was a "normal child" to begin with - and with a lot of disabilities, that's simply not true.
When you decide to have a child, by whatever method you choose, you are not choosing to have a nondisabled child. Even adoption is not foolproof - there may be a disability that is not apparent just yet. There are no conditions you can invoke so that you can avoid the dreaded disability. You are choosing to have a CHILD, period. If your child has a disability, don't get upset because you didn't get what you wanted. Your child is not a restaurant meal - you cannot say to whatever babymaking deities you believe in, "I didn't order this" and return it to the kitchen. You have to go along for the ride - for better or worse. Grieving a "perfect" child that never existed will do neither you nor your child any good.
What are we telling disabled people when we tell them that their parents grieved as though there was a death? What are we telling them when this process is actively encouraged as a form of catharsis? Is it any wonder that Tim Bowers chose to end his life one day after learning that he would be permanently paralyzed? In our society, disability is a fate worse than death, and disabled people are living ghosts.
I don't want to be a ghost. I want to be a person.
If you're a parent, and you've felt that grief, it doesn't mean you're a bad parent, either. But it does mean that you need to think critically about why you felt that grief. What did you know of disability before your child was diagnosed? What are the portrayals of disability you've seen in the media? When the doctors told you, what did they say? Did their tone convey solemnity, as if telling you of a loved one's death? Were they afraid to say the words? All that and more contributes to the idea of disability you form. Ideas, as any sociologist worth their salt knows, do not exist in a vacuum. They are influenced and shaped by everything around us. Take a step back and learn from your children, who are undoubtedly still happy, despite the Big Bad Disability. Remember that disability is not a deficit or a cruel twist of fate - it simply is.
To parents, I say - it's okay to be sad. Sad that your child will face a more difficult path than most. Sad that the world may never see them as full human beings. But it's not okay to grieve. Because nobody died.