Those ages can be quite a spread when you're having a discussion!
For the little two, could you start with teaching them *how* to have a discussion before you hope to have actual good discussions with them? Do you do morning time with your kids together? That would be a great time to practice your discussions. Pick something really simple to discuss that the 5 and 7 year olds would have a lot to talk about. Think about the behaviors they're displaying, and which ones need to change (talking over others, interrupting, chiming in with something completely irrelevant, silliness, etc.). Remind them of your expectations and focus on one or two changes at a time. Let the older kids know what you're doing and what the goal is, and recruit them as role models.
It might help to have a tangible, physical reminder of whose turn it is to speak. Perhaps some variation of the "talking stick" to start out? A ball to (gently) toss to the next person to speak? You could also give them a sentence starter to help guide them. When they are responding to someone they have to use one of your sentence starters like, "I agree with Johnny because...." or "I disagree with Johnny because..." That gives them some boundaries. Once you feel like they have a grasp of the boundaries and expectation, you could move them toward most substantive discussions.
With the older kids, I'd try to have discussions just with them; make it a little more grownup. While the younger two are playing somewhere else or resting, gather the older two somewhere quiet. I have boys, and I always try to have a food item available to help encourage the idea of gathering somewhere, having good food, and talking. Usually it's cocoa and cookies. Maybe have some soft music in the background? ("Epic soundtracks" is a great Pandora station for boys! ) If you're having them help the littles practice discussing at another time, they'll know your expectations and you can hopefully have a more productive discussion with them.
I agree that with essentially 4 grade levels you can't have every subject he a Socratic discussion, or you'd never get anything accomplished. But maybe start with one that you do in pairs or together and then sprinkle others in over time?