Ask questions. Listen. Understand the current owners' long-term goals. Share your goals and new passion for acquisition. See if the topic comes up naturally.

Some great advice here already - easier since you already know them and I would stress the importance of the personal relationship and caring about them as a person: I don't know this company, but lots of owners get pestered by financial buyers who only care about the money and have little care about the personal side or knowing the people within the business. If you want to stand out - do it on the relationships side. Further, over time you will figure out what your personal pitch is for why to sell to you - if this one doesn't work out, practice it as much as you can with other targets