What is the right way to discuss that at an interview? Discussing the applicant's family is typically off limits, but if the goal is to work and be a parent then you must have that conversation early.
You shouldn't discuss family obligations at a job interview, full stop.
I think if you are doing child care and work at the same time it is relevant. If somebody was taking their child to the office, however, that would relevant.
It's relevant, but it's illegal for a prospective employer to ask about and it's against a prospective employee's interest to raise the issue.