Our census has been fluctuating a lot lately. People in overtime and PRN folks are usually canceled first. We keep a list of who gets canceled, put on call or floated and rotate through the nurses. If a nurse is put on call, especially for a night shift, we only keep them on call until 11pm, even though I think hospital policy allows up until 3am. Every unit I've worked on at this hospital doesn't call someone in after 11 or 12.
Our full-time charge nurses never get put on call or canceled unless it's an extra shift for us. There are only 4 of us (2 on dayshift, 2 on night shift) on the unit. Relief charge nurses are rotated through like all the other nurses unless they're the only person trained to charge on the low census day.
If you like your job, my advice would be to get a PRN/flexi job to supplement your income. If not, you could always try moving to another unit that has a more stable census. Before accepting the position I have now, I interview for a charge nurse role at a smaller hospital in my area. On the day of my interview, I noticed nurses were transferring patients and cleaning/bagging equipment. I asked what they were doing and was told that the unit was closing because the hospital census was low. The nurses and nurse manager I interviewed with were very nice, but I ended up passing on the job offer because I needed a more stable income.