As for suggesting to the kids that they stop kissing, I think that you can approach it indirectly. Finally, I think that follow up is essential. I try to look for opportunities to explain these types of issues whenever they come up. I don't dwell on them and frankly let my daughter dictate how much is discussed. When she seems to have heard enough, I stop and wait for another opportunity.
Kids seem to be self-regulating on most things.
This is not a situation I anticipated handling with my 4 soon to be 5 year old daughter and have no idea how to handle. A 4 soon to be 5 year old boy lives on our street about 5 houses down on the other side and attends the same preschool our daughter does. He lives in a single father household and has seen his mother 1 time in the past 2 years. His maternal grandparents cut contact with the boy 18 months ago or so. The boy's father does a remarkable job and is a very good father.