An old moan: women with small children end up taking all the flexible working hours (because there's a limited amount to go round?) and when they are absent or work shorted hours their workload is dumped on non-parents. I'm sure this happens - though I'm disturbed by how often it's used as another excuse to bash parents (usually mothers) in paid work, rather than an issue with the organisation and structure of the workplace.
But my experience has been just the opposite. Particularly in my current workplace, I've found working with people who have small children leads to an understanding that (a) there are other things in our life that need attention and (b) the measure of good performance at work is not clocking in at 8:30am and out at 5pm. Have I covered for people because of childcare related needs? Sure. But I've also been granted flexibility and/or had people cover for me when studying, or when I was house buying and needed to pick up documents and once when I came in two hours late because I couldn't find my shoes.
Of course my experience isn't universal - it's not even universal in the history of my own employment (I've worked in call centres. Yeah.) but I want to get experiences like this out there not so much to counter what people are claiming to be the case, but to show what can be the case, and in doing so focus on what the problem is and isn't. And it isn't people (and we know that usually means women) having kids.