From a hygiene standard, I don't worry about it. There are very, very few diseases that can be transmitted from dog to human, even though their mouths are disgusting. I also mentioned it to my daughter's NICU nurses when we were getting ready to take her home, and their response was basically "<shrug> just don't let the dog lick her too much."

Dogs instinctively lick the mouth of anyone senior to them in the pack, so the dog licking your kids faces is just their way of showing they're submissive.

However, one of our dog gets so aggressive with her affection that we've stopped letting her face lick because she kept knocking the newly-sitting baby over or licking her so much LO basically couldn't take a breath. We just redirect the licking to hands or feet now.