If this is a family member I would just sit down and have an up-front conversation about it. "Hey, you mentioned using me for back-up child care in emergencies. I'm happy to help if I can, but can we set some ground rules so we both know what to expect?" Then figure out what would feel fair to you given your responsibilities to your own family. Do you want advance notice first? Do you want to impose a limit to the number of days (e.g. once per month) you can cover for? Do you have to maintain a specific schedule? etc. Pushing back a little to establish some boundaries might help this person understand you have your own responsibilities and that your time is valuable too.
Reply to SAHM: So you can watch my kids, right?