I'll go against the safety thing. One cool thing (I have a degree in aerospace engineering) is the fact that you can basically make rocket fuel out of your favorite junk foods (oreos and jolly ranchers work pretty well). Obviously do it with an adult. You can fairly easily test how high a given rocket will go with a thing of kite string. Also you'll notice that you can easily determine a comparison between how high the rocket goes vs how much calories the food has.
But onto more realistic science experiment ideas:
1-Renewable energy/efficiency is def in - windmills, solar cells, insulation values. Perpetual energy machines are always interesting, especially if they can understand why they dont work.
2-Basic physics - its easy to do with a few marbles a piece of wood with a track cut in it, and a few blocks to make an incline, and a stopwatch.
3-How fire alarms work is actually very interesting
4-If they're mathy you could have them go over crypto type stuff (just think you can teach them to write notes to their friends without you being able to read them). The RSA algorithm requires only the knowledge of what a prime number is and how to do long division and multiplication to make it work. Completely safe otherwise. All you end up needing is a calculator or excel
5-Color of lights that helps plants grow
6-Fitness is good and very in vogue, if they're into that you could have them start an exercise program, and chart their resting heart rate every morning. Alternatively you could do body fat %age, abilities (run 1 lap around a track), etc. Depends on how much time you have to see actual results. If one wants to do it and one doesnt you have a control. Alternatively one of the parents could volunteer to be the control or subject or etc.
7-Aerodynamics - how a plane works, why a dart always lands tip first regardless of how its thrown (assuming you have a far enough distance to the target). Bouyancy, how boats float, etc.
I could go on for days. I wish I enjoyed science fair type stuff this much when I was a kid but I'll stop being annoying