So now, here is the child's problem
I have used restricted language myself, for the same reasons outlined above. It's kind of a "just this one time to get things done," situation.
The problem is that because it is successful we tend to use it more and more (kind of a Skinnerian or operant conditioning effect I suppose).
We may use it in the morning to get everyone off on time; and we may use it in the evening just to get supper, clean the house; and in between to keep our sanity.
I think only a grandmother or grandfather can appreciate how unbelievably quickly the weeks (months-years) sneak by while your doing this. And without realizing it, this has become the linguistic style of the child-restricted language.
In essence, the child does not have a decoding system that deals well with many syntactic nuances. He/she finds these tedious and incomprehensible. This does not set the stage favorabley to have good learning experiences in school.