It's not discriminating against familial status--it's acknowledging someone's expectations about being allowed to miss work. Someone with the same familial status who arranges for other accommodations for getting their kids to and from school would not be affected.

Sick leave policy is up to the company. You cannot assume you are entitled to more sick days simply because you have children. If you will be taking off additional time due to having to care for sick children, that should be discussed ahead of time.

Emergencies do happen, but that doesn't excuse poor planning. A child becoming sick is not unforeseeable, and it's not unreasonable for both parties to acknowledge what the expectations are.

Many (most?) companies absolutely do make for allowances for employees with children, but that doesn't mean employers should be on the hook for whatever expectations the employee has. This is absolutely reasonably in-scope for an interview.