My 6 year old boy enjoyed Snap Circuits. He has two sets. One with a "green energy" theme, and another with a spinning fan thingy that shoots up to the ceiling.
I'd say at the conceptual level, he has learned that circuits require a closed loop, and he has certainly gotten good at making the motors and lights turn on. He has learned switches. He has not really grasped anything about energy: charging, discharging, etc.
I had a spring-based 160-in-one as a kid and enjoyed the heck out of that thing. However, based on what I've observed of my son, it would be too soon to get him that sort of kit.
He also has been given an electromagnetic exploration kit that included a spool of magnet wire and instructions to wind various solenoids. This one, too, has required a lot of parental intervention because the instructions were a bit difficult for him and because he's just not that coordinated for the fine motor skills. But we did make a cool little motor with a spool of wire with the ends de-shellacked on one side, suspended by two paperclips.
Honestly, he has a lot of fun with just batteries, some wire, and various LEDs and other bits and bobs he steals from my bench.