I've waded in at the deep end with some topics for my kids.
We've covered orbital mechanics with much success and kerballing. Covalent bonding was pretty good too, using Lego as an analogy. AC vs DC electricity, with voltage vs current was okay too.
Of course, all these are superseded by discussions of Nintendo vs Sega, and why Nintendo were crazy (but ultimately justified) in making the N64 a cartridge console, 13 years before my kids were born. There's only so far you can go before their attention goes elsewhere...
Edit - to be clear, my kids steer the conversation to old consoles, not me!