Unfortunately, people that don't comply with the requirements / recommendations become everyone's business because of the virus. I wouldn't confront anyone directly over this but depending on the situation, notifying a CM is not out of the question. I think the best recommendation here is to take the child to one of the designated relaxation areas so that they can nap without a mask and not violate the rules.
This isn't about someone walking around without a mask (which is an obvious violation of the current rules), this is a small child, sleeping under a blanket or other covering to the point that no one can even see the face of the child cause it's "covered". I can't fathom reporting something like this to a CM, my job is to enjoy my vacation, not try and overly enforce rules on other guests. There are CMs doing that already.