My first instinct is that if your son in law thought he was doing something wrong or had questionable motives then he would never have mentioned it to his wife. You need to be very very careful about these sorts of "almost" accusations. If my husband was put in a situation like this with my mother then that relationship would be over before anyone realized what happened. Your concerns could easily be interpreted as a threat to the security of his family... Men aren't usually calm about that sort of thing.
All that being said, of course part of your job as Gma is to help look after the safety of your grandkids. It sounds like your daughter and son in law encourage curiosity and frankness in regards to their children and their bits and pieces. Not feeling like those things are dirty or secret is one of the best ways to keep children safe from predators. The unimaginable might still happen because guardians can't control everything all the time, but hopefully the child will know enough and be comfortable enough to tell someone. I think, in this situation, the best way to do your Gma Job is to be supportive of their parenting. It could be a little awkward at first when your granddaughter comes to you and announces that she has an "A" and Daddy has a "B", but your response will either encourage or discourage her from feeling comfortable talking to you about that kind of thing in the future. Also remember that more than likely, once she is a teenager, she will not feel comfortable talking to her parents about sex and it would be wonderful for her if she had a Gma she knew she could talk to. THAT would be a great time to introduce some of your traditional values regarding sex