So... one thing I can say from experience is that you need to be aware of the motor skill limitations of the under 12 set. There really is something that happens around age 12 that causes a quantum leap in fine motor skills. So... tune your projects to minimize frustration around that. I learned that the hard way by seeing a selection of my projects randomly fall one side or the other of that motor skill limitation because I was clueless when I designed the experiments.
Now to your direct question: Once upon a time I did a 10 week basic electricity class for about 8 kids of about 7 years old. Yes, it was pretty basic. One thing I took advantage of was that Harbor Freight was running a deal where you could get a multi-meter for US$3. So... I gave one to every kid.
First lesson was "conductors and insulators" -- I can't remember how I explained the concept to 7 year olds, but the lab was a hoot. I had a bucket of stuff like nails, bits of cloth, an apple, etc, just random stuff. And a dish of salt, and a dish of water. Use the Ohmmeter to find out what is a conductor and what is an insulator. After everyone had discovered that the dish of salt and the dish of water were both insulators.... I poured some water into the salt. Woah!!! It conducts! What's up with that? By the end of the lab I had totally lost control and the kids were raiding the fruit plate in the kitchen... "Is a banana a conductor?" ... followed by vigorous stabbing of meter probes. Anyway, the teachers you are working with may not be so cool with the losing-control-of-the-class part, but I count that as a metric of success.
Another lesson that was very popular, but required too much motor skill, is that I had rounded up some 7-segment LED displays, and some 8-gang DIP switches. (We had built a single LED circuit in a previous lesson.) I had them build up a circuit where they could control each segment with it's own switch (hint: prototyping boards are beyond 7yo motor skills...we had enough parents to help with the wiring...) After it worked, I had them make numbers by setting switches. Then I asked them to invent other displays, like find "letters" or just fun shapes. Then... I told them to look around the house, and their own house when they got home, to find number displays. Like on the microwave oven, etc. You could sense the real "Wow, now I understand this!" moment when they realized the basic operation of a common part of their world.
Bottom line: If you can find a way to help them understand even just a little bit of how their world works, it will be a popular experiment.