For example, how to communicate the complexity of political systems?!?! This started with a conversation about pharaohs and dictators prior to viewing the musical Joseph and his Technicolor Dream Coat. Kids tend to be very into making their own choices with all that budding independence :) I said that in a country with a dictator, the dictator got to make all the choices. In a democracy, most of the people or the majority got to make the choices; it didn't matter what each person wanted. In a republic with rights rules, each person got to make their own choices as long as they didn't hurt anyone else. (My husband did something similar with economic systems regarding decisions on property. )
I left out everything about mixed cases, elections, constitution, congress, courts, military.
Eventually we delved deeper into how many of those other topics work and he has come back to the subject often. He doesn't want to visit anywhere that doesn't "have rights".
Again, I think the key in communicating a new concept is providing a graspable chunk. If my son can't get that chunk, then he's not going to come back to learn more. The whole opportunity will be lost in a fog of details (sinking concretes) and nothing remains retained.
I think Scott Powell does a superb job of recognizing this challenge for anyone learning new material in his History At Our House curriculum and I'm thoroughly enjoying learning more from him.