Sammy's mother explained to him that the family would take a vacation in Miami. The next morning, Sammy announced, "I gotted my bags packed. When are we going to Your-ami?" What explains Sammy's errors?

I believe the answer is: it seems that Sammy has early signs of literacy problems, In the scenario above, sammy probably mistook mi-ami into my-ami.

So rather than thinking about the name of the city, he's thinking about the place that belonged to his mother in some way.