Often times a potential employer will ask about your availability to see if it suits the needs of their organization. Being a plumber will often mean 24/7 on call for emergency plumbing needs. How do you feel about this?
If you have all the flex in the world with your schedule, you could reply: "My family understands the potentially late or random hours required of a plumber and we are all prepared for that commitment."
If your schedule is not as flexible, you could reply: "I have been on call from time to time throughout my career as a plumber. I am willing to be a team player and do my share however; my family prefers that I am not on call all of the time. How often would you require me to be on call?" You can chat further from there. Perhaps they only ask for one weekend per month from you, etc. Be careful not to rule out the position entirely before finding out all of the details.
"My family understands the potentially late or random hours required of a plumber and we are all prepared for that commitment."