Holly

I second this- starting from a place of love and concern for the parents, rather than what they are doing right/wrong as parents. I also think coming across as part of their team, rather than a critic can help. (BTW it sounds like you are an amazing granny- you’re here, educating yourself and asking this question out of love, hope this one goes over well!)
Here’s an extra little script offering, take or leave it: “I love you and I feel worried about your well being with sleep. I have the bandwidth to help right now(assuming you do), would that be okay?”
See how they respond! If it’s a no, it’s a no. If it were me, I would burst into tears and tell you how exhausted I was and let you in to offer some fixes. Good luck 💜