Personally, I wouldn't talk about this to people other than the child's parents. Disabilities are widely misunderstood, just as a child's difficulty, even failure, in school is widely misunderstood.
There are different possible reasons why your granddaughter isn't speaking clearly yet. I don't even want to list them because, what would be the point? Let the people who are trained to diagnose her deal with it, and you can deal with it when they get a diagnosis. All this talk about her being "retarded", which is correctly referred to as "developmentally disabled", is premature. There are other possible reasons than IQ. What is happening here is that professionals, like her pediatrician, are trying to catch any disabilities, or other issues, early so that they can be dealt with, before she enters school. That way, when she does enter school, she has a better chance of being successful.
If her language development isn't as far advanced as with other children her age, she can receive language services that will prepare her for school. Good grief, she could just be immature.