I think it's fantastic you have these conversations with your girls! When I was growing up, we had dinner conversations that were similar. As the youngest in the family, it was sometimes hard for me to keep up, but my parents never dumbed it down for me - they just explained it in words I could understand. Then they asked my opinion. 

What I didn't realize then was how much I learned simply by absorbing the conversations. Being included in "deep thoughts" pushed me to think beyond my physical age. I can honestly say it's helped a great deal, both personally and academically. Your daughters are very lucky!