Good luck and please be careful when taking them apart especially the flash models as the capacitors can easily discharge if you touch the circuit board even if you have taken out the battery. Discharge the flash by winding on and depressing the shutter and taking out the battery quickly after, you can also use a insulated metal object to bridge the capacitor as well. Take them apart after you are finished with the roll as your film will be rolled in, its much harder to do it in the dark or a bag on an unexposed camera.

Some do have a teeth/gear style center post which makes using regular reloadable cassettes impossible. You can ask your photo lab for canister discards. You can easily tell them apart as the have very plain solid color text logos and have the toothed gearing on top.

Pinhole photography is very easy to play with, and very cheap to get started. Just a tube or a box and metal or foil to poke a needle through, having a darkroom though is important. (unless you build a afgan camera pinhole)

Keep things simple to start out if you are focusing on composition. Things like keeping subjects off center, watching out not to cut off top of heads, or hands, or if they want to, tell them where would be a good place to (not at the wrists or ankles lol). Teach them to use the whole frame up from top to bottom. If using the disposables make them aware of parallax and show them a diagram of both field of view from viewfinder and lens on upclose subjects. I do this when using compacts, will save you trouble later haha. Good luck!