You can never be too careful.
Many of these experiments are not for the young. In particular, neodymium magnets are very strong and can hurt small fingers.
I know I don't need to say this .. but .. Eye protection is a must.
In particular, neodymium magnets are very strong. I have personally been hurt many times playing with these. The rule is (has to be) - keep unused magnets stuck to something if you are not using them. If they are just "sitting on the table", they will fly thru space and smash your finger anytime you move another magnet "too close". (The definition of "too close" is a bit ambiguous - if your finger gets smashed, then you were "too close".) Neodymium magnets are also a bit brittle, I have broken many magnets by "letting" one fly into another. (Another definition of "too close".) I expect you to also break a "few". (Which, of course, explains the rule to always stick unused magnets to something.)
When intentionally putting magnets together (which I do a lot), be very careful. These things are strong and will simply fly out of your hands. Also, beware the blood blisters. If 2 magnets get too close together without you being in complete control, they will either fly out of your hands (and probably break) or they will catch a bit of skin between them .. and crush it flat. Yes, it hurts .. a lot. (You've been warned.)
I have a 7/8" neodymium that I use a lot. After telling a person to hold it tight, I have seen it fly out of an adult's hand. The force is surprisingly large. If you don't understand its strength, you will not be able to control it.
In particular, young children should not handle magnets without very close adult supervision. It helps if you've already been hurt a few times so that you understand what to watch out for. With small hands, and big magnets, the risks are very high.
This is a youtube video showing how dangerous magnets can be.
Cutting Soda Cans
Even if you are careful, it is very likely that you will cut yourself.
As a result, I suggest that you wear leather work gloves when cutting up a can.
Once the can is cut, you should use a "file" to smooth all the edges and to round off the corners. (For a "file", you can use a real file, a fingernail file, a coarse rock, emery paper, and so forth .. as long as you make the objects safe to handle.)
Be sure not to use the "good" scissors to cut aluminum cans. The danger from doing so is far greater than a simple cut. I have a special set of scissors for this task that no one else in the house uses. Even so, I still get yelled at when someone sees me using scissors on metal. (You've been warned.)
It is tempting to add more batteries to make the screw-based homopolar motor go faster. (Yes it will.) Don't do that without full face protection. These things can get pretty scary.
I develop homopolar motors using batteries that are partly dead. This is a great safety idea. Most people don't realize how much energy is in a battery .. until they do something that scares them to death. Trust me on this - always think safety .. for yourself and those unlucky enough to be around you.