1. Air up tires at least weekly, and check daily. The "squeeze test" will indicate whether or not further check is needed.
2. Fixing a flat -- have the tools and supplies, they will fit in a seatbag. Spare tube, patch kit, tire levers, proper tool for wheel removal.
3. Lube the chain. How to do it.
4. Adjusting saddle/stem height (on threaded-headset bikes, the proper bolt to loosen/tighten).
5. If derailleur-equipped, teach them WHAT TO LEAVE ALONE until they get better at it!
6. Ride WITH traffic. Learn and obey laws.