I am childfree by choice, and in the majority of jobs I've had over the years the assumption would always be that I would be flexible round any mothers in the team, which I was as much as I was able.
I would be the one to stay late if it was needed as I didn't have to pick up/feed the kids.
I would always be the one to come in early if it was needed as don't have to take kids to school.
I would cover at weekends if it was needed, as I didn't have kids to look after and entertain.
I would take my holidays in school term, 'cause no kids so I could.
I would work over the Christmas period, because Christmas is for kids.
I could rearrange any plans I had, because some mother's child had their school play or sports day.
And it often wasn't the management, or the organisations. The sheer pressure I would get about how selfish and mean I was not letting them take priority ALL the time (including being totally isolated at work as a "punishment" at one place),. In one place I really pitied my manager as if for any reason at all I didn't change my plans for a mother they would pester the manager non-stop, report her (and me) to HR for discrimination in one case (!), go on a "work to rule", and make both our lives hell.
I would do my best to try to fit in around their plans, and usually even volunteered to cover the "awkward" times. But things like spending Christmas away with my cousin & my aunt/Godmother because it was likely to be her last? I kicked back on being asked to change that. And being asked to cancel my (already paid for) holiday away with a close friend who was going through a hard time because a mother "couldn't" miss her child's sports day, and getting pressure from my manager to give them an easy life? That one was the last straw for me.
When I was a manager I would do anything within reason to accommodate parents, as I would with any other requirements like religious needs etc. I arranged meetings so they would never be on a Friday afternoon, as a devout Jew in my team needed to be home before dark. I would allow a mother to take a very short lunch break and an extra long period in the mid afternoon to collect her child from school etc.
And the majority of staff were grateful. But there were a couple who would take the piss big time, both mothers. They would never be on line before 9:30 am (school run). They would log off at about 2:45 - 3:00 in the afternoon (school run) and if I was lucky they would log back on at about 4:15 pm, but would be almost impossible to get hold of (if challenged, would be giving the kids their tea). And of course would log off for the day at about 5:30. And yes that was taken further, but both resigned during their performance improvement periods (one right at the start, one the day before we were going to sack them).
Don't get my wrong, I've worked with some absolutely fantastic working mothers who I would do almost anything for, to make their lives easier. But there have been some who seem to think everybody should fit round them & they should be able to get away with murder because they have children - I'd guess from my experiences that's probably around 1 in 10 who are so entitled.