M.S.
I am so sorry for your loss.... I would probably have the same concern... (I am paranoid and am always considering what someone else might think). I think it's great that you are handling this so well.
If I were you, I would simply talk to your friends and tell them what happened, but that it will in no way affect the job you will do taking care of their son. Just discuss your concerns with them... I'm sure they will be relieved if you talk first, because it's so hard to know what to say when someone loses their child. (especially if you have a child... you feel like you should avoid just in case the mere presence of a child will upset, etc...)
Good luck!