Perhaps you could find another way to get your point across without resorting to childish name calling that only hurts those that cannot defend themselves.
Iforget:
Having worked to secure funding for services to this population, I am familiar with the evolution of the politically correct terminology - "a child with a developmental disability" is the most current (they are a person first).
The word retarded is technically accurate - it describes the path of physical and mental development. However, that word is no longer used in polite society to describe one who has a developmental delay or disability. (I believe the word retarded is in use among clinicians, simply because it is accurate.) The point is the word 'retarded' was hijacked into common slang usage as a disparaging description - leading to its declining use as a clinical term. The word now belongs primarily to this usage, as many perfectly good words do (such as "gay" or "queer." "Moron" and "imbecile" are both valid clinical terms).
As you travel this road with your child, your feelings are raw and exposed, making you hyper-sensitive, it will be important to develop a great deal of tolerance and a bit of a tougher hide.