Ideally with should always be flexible. However if it’s not there should be a good reason for exceptions and having to care for a young child is often a good reason in my opinion. I believe people without children can have good reasons as well and some people with children need less flexibility. E.g. a single parent usually struggles more than one with a partner stays home to focus on the children or one who lives in the same house as the grandparents. If you’re speaking about vacations it’s usually due to fixed school vacations so if the person with a child wants to be able e.g. wants to travel with their family they have to do it during the official school vacations. At least here school is mandatory otherwise and not sending your child to school is a crime.