Speaking as a (woman) academic whose career has slowed since motherhood, I have found that the slowing down has happened gradually since I had my children, and has accelerated since they went to school.
Initially, I would say that even though I chose to come back from work after my first maternity leave on a fractional (0.7) basis, it didn't affect my research career. I was able to draw on projects started before I became pregnant, and on contacts and networks that I made when I was able to go to many more conferences and so forth. And my childcare was tailored around my needs as a working mother.
Once I had another child, and having two (and one with increasingly apparent SNs) was much more time-consuming than having one, my ability to draw on work I'd previously done as the basis for new research was diminishing. I was doing fewer conferences, and said no to more things because I couldn't take them on. The direction of my research changed (and I suspect this is an arts thing) because of the change in my life and thinking my children had meant.
Then they went to school, and suddenly the childcare wasn't tailored around my needs. One child's SNs meant that we couldn't use after-school care: school was enough of a burden for him. My ability to find stretches of time to start new research projects was severely affected by childcare time. As they have got older they don't need me less -- indeed, in some ways, they need me more. And I can no longer count on them being in bed and using the evening to work in.
So: the networking/conferencing side of research life has been eroded, and the stretches of time necessary to do new reading and major writing have been whittled away, and I can no longer build on what I was doing 10 years ago -- and my research has now settled in a much less fashionable area.
I wanted to tell that whole story because it's not just what happens to your career in the immediate aftermath of having a child that counts, it's what happens to it in the years after. As it is, I am ok with knowing that I'll never be promoted, and I am happy that my childless colleagues do get the rewards that their dedication gets them. It's just that having children is a long game, and the times when it can make the most demands of you are often those middle years when you might be expected to be making your career.