You did exactly the right thing , and you know that. So just behave normally towards the family and get on with your life.
You have spared a child years of unhappiness and maybe even saved its life.
There could not be a finer thing to do than that.
The parents may think it was you , but so what, admit nothing if you are challenged by them, say nothing to anyone about this, and definitely put nothing in writing.
You are in the right here, and let no one try to put you in the wrong, by funny looks , or not speaking to you.
The bottom line in all this really is --just how important to you is it that the parents speak to you.The child is important here as you have recognised, so get on with your life, and be proud of the action you have taken.
Best of luck . B renda.
P.S Remember you are not the onlyone who can hear things as has been mentioned , and children also can pass things on to there little friends , so forget the whole thing now.