I do an open English Discussion Club which attracts all sorts of different age groups and levels, not to mention completely different students every time. What I usually do is try to generate a wide level of questions and go from easier to more difficult.
For example if the theme is happiness, I might start by asking,
"What makes you happy? What makes you unhappy? What's the happiest time of year?"
Then later, I would get into, "What is happiness? Is it just an emotion caused by chemicals or is it something spiritual? Is happiness important? Is it true that it is better to be happy than rich?"
This helps target everyone in the class but it also helps to encourage the lower level students along. If you start easy and build their confidence they are more likely to keep trying to participate even as the discussion gets more complicated. Also, even easy questions can have complicated answers so the higher level students usually don't get bored.
I think it is also vital to make the class a safe space. You don't want the higher level students mocking the lower level students or correcting them harshly. I would stop that immediately as soon as it happens, and also validate good but maybe not accurate answers. When beginner students say something like, "The cook make happy me," I try to smile and say, "Yes, cooking and food are very important for happiness." or something to make them see they were understood.
Hope that helps a bit, at least.