Just adding that I think it really, REALLY depends on the situation and the individual child.
I have cared for a child in the past that I am absolutely certain had/has a sensory disorder. I waffled on telling the parents. I did not in the end, only because although there were some "quirks" to his behaviour, I did not feel that he was not progressing.
One of the hardest things to deal with in this job can be drawing the line between what WE deal with on our own, and what we need to turn to the parents with, and ask them to address.