Would you be available to work extra hours if needed?

Your willingness to be available for extra hours in certain circumstances. Again, you're not offering up your whole life, so imagine a scenario that involves a one-time project where the deadline was unexpectedly moved up with no notice to the company. The supervisor asks if you could stay an hour late (or come in an hour early) 3 days this week to ensure the project can be completed.

Saying that you have kids (or dogs or whatever it is) that you need to be home for at a specific time every day. That may be true. But don't say that. Let the interviewer know that if specific occasions arise once a year where you are asked to stay late, arrangements could be made. Again, they're not asking you to work a million extra hours, just maybe once in a while (if ever) during a particularly stressful time.

"Sure. I do have other commitments outside of work as we all do, but with a few hours' notice I can arrange to stay late/come in early/work on a weekend if necessary."