Kudos to you for your efforts to educate youngsters about the universe! The knowledge they gain, and the enthusiasm you bring to them that leads to more knowledge in the future, will help them become better citizens in a society based on use of technology to provide a better life.
All the kinds of activities you mention are good ones. They touch on basic astronomical observations (sunspots; paths of the sun and moon across the sky), basic physics (gravity, and how things fall), mathematics (geometry: big things far away look small).
This points to the key to astronomical understanding: we can do things here on Earth to understand how the universe works, learning about fundamental concepts in mathematics, physics, chemistry, etc.; we can observe things in the sky, such as the moon, the sun, planets, stars, how they move (or appear not to!), etc.; and then by applying what we know about how the universe works to the things we observe, we can learn more about those things in the sky than the simple observations alone could tell us. You've started that process by bringing in some fundamental geometry, fundamental physics, and simple observations.
This is a great direction to take. You might add some simple chemistry: add some vinegar to baking soda, and from a liquid and a solid you can get a gas; add some vinegar to milk, and from two liquids you get a (somewhat gooey!) solid, with some liquid left over. Age 7-8 isn't too young to start with the concept of atoms and molecules, and that all matter you can see with your eyes, or feel with your hands or fingers or cheeks (like wind), including the matter you can see in the sky, is made up of them. With these simple kitchen experiments you can show that sometimes, when molecules come together, the atoms in them rearrange and make new molecules that can be very different from the ones you started with (the basic concept of the chemical reaction).
Binoculars suggested in a comment by @DonBranson are also a good start. The moon is always a good first astronomical target. Start by going to a sandbox with some marbles, or a golf ball, or small rocks—whatever!—and show that objects falling onto a surface can make craters. Then with some ~10-power binoculars (they don't need to be expensive) you can begin to see some of the larger, well-formed lunar craters like Copernicus or Tycho, when they're near the terminator. This reinforces the idea that things you can do here on Earth can help you understand things you see in the sky. Just be sure the kids know NEVER to point them at the sun!!
With a pinhole camera you can show sunspots. First use it indoors, maybe with a flourescent lamp or some other lamp whose shape is not circular, and show that the pinhole camera makes an upside-down and reversed image of the light source. Then go out and use it on the sun.
There are several web sites treating astronomy for children. The National Science Foundation has one, NASA has one, JPL has one (though it appears to concentrate on somewhat older children), there are some not associated with any government agency (this for example). The Kern Astronomical Society has a page with links to a variety of such web sites. The material at these sties might or might not be useful for children to visit themselves, but from them you might get some good ideas for in-home, outdoor, or night-time activities.
But I can't overemphasize the value of one more thing:
A telescope!
My wife and I are both space exploration professionals, and we know a lot of such folk. When we discuss what things made us, as children, take an interest in things astronomical, the most common theme is either having, or having access to, a telescope.
It doesn't have to be expensive at all. For children, you don't need a 5000-buck, 20-cm (8") Schmidt-Cassegrain or Maksutov with clock drive and computer-controlled pointing. You don't need a concrete-mounted post or a 300-buck tripod. You just need a 4 or 5 cm (1-1/2" or 2") (objective diameter), 30- to 50-power cheapie refractor, on a simple short pedestal or short tripod you can put on a TV tray in the back yard. With set-ups like this I've seen adults draw in their breath when they see lunar craters near the terminator for the first time! You can see the moons of Jupiter. You can resolve stars in the Milky Way. You can see the double star in Ursa Major, or the double-double in Cygnus. Once you've shown a curiosity-filled youngster a few such things, on their own they'll want to point it all over the sky, looking for new things. This is a great parent-child or mentor-child activity.
Of course telescopes can be used for other things as well, and looking at ordinary, every-day objects can help teach a child how the telescope works.
Just make sure they don't point it at the sun without a sun filter!
Without a clock drive you can point out that things appear to move in the telescope's field of view, and this is because we're not on a motionless platform: our platform, Earth, is rotating. You can do an indoor activity using a swivel chair to demonstrate how things appear to move if you're on a rotating platform, and then show that the sun and the stars appear to move in the same way.
One caution about the telescope: adults can get hooked too. After a few nights with the cheapie they want a better view of Saturn's rings (the cheapie will only give a tantalizing hint of the rings' glory!), so now the 8-cm (~3"), chromatic-aberration-corrected refractor on a full-size tripod doesn't seem extravagant. Then the clock drive. And if you're going to spring for a clock drive, why not get one with a 20-cm Schmidt-Cassegrain attached? There are many avenues for spending lots of money on telescopes and accessories.
I hope this is helpful. Best of luck with your mentoring! And thank you for your efforts to make our society better.
