I think it depends very much on your field, how competitive it is and what the usual working patterns are. Also what you want and expect from your career.
I used to have a (full-time, permanent, ie. v. rare) post as a lecturer in a not-terribly-distinguished university, but with good PhD, good contacts and excellent prospects. So there I am, aged 28 with an eighteen month old dd1, and it slowly dawns on me that to make it big in the academic field you have to a) be prepared to move posts right across the country every few years b) spend a lot of time bidding for research funding, doing research, writing it up and presenting papers. And this is in addition to doing the day job of lecturing, teaching, preparing and taking a lot of crap from the powers that be. For this you get to earn somewhere in the region of the national average wage. Rather belatedly I realised that this career was rather less than the sum of its parts, and that the parts couldn't all be added anyway.
Now, ten years later, I do consultancy work in a very specialised subsection of the field I used to lecture in. There are fewer than half a dozen people in the country doing this work, so I can mainly work on my own terms. The workload is more interesting, more varied and means I can be there for the children (which is actually more important, not less as they get to school age). I also earn approximately double what I did as a lecturer, and could increase it if I had the time to put in extra hours (which I don't atm).
From one angle, I'm on the Mummy track. My publications record is pretty pitiful, I don't go to many conferences or schmooze. Ironically, if I were to apply for a lectureship I wouldn't stand a chance against all the bright young things with shiny PhDs fresh from the conference track. But on all other counts, my current situation wins hands down on my previous one.
Moral: carve out a niche for yourself where you get to set some of the rules. It must be exceptionally difficult to compete as a mother on level terms with ambitious childless late twenty-somethings. But if you are v. good at doing what you do, and there aren't hundreds of other people who could do it just as well, then I don't think doing some time on the Mummy track needs to be a career killer.