I'm possibly coming in a bit late here, but have been too busy wrestling my 12mo to write!
I'm coming from a slightly different perspective, in that I started my PhD with 2 kids already, having been made redundant from a career in consultancy. I had hoped to have number 3 shortly after completing, but had to take time off early in my pregnancy when my husband was in intensive care with swine flu. I went back to it when DS3 was six months, and am due to submit in a fortnight (slight panic...)
Anyway, my supervisor has been off twice having babies in that time. I've developed a strong relationship with my other supervisor (they're sort of equal first, rather than one being a second supervisor) and the one on maternity leave has come back in for meetings when she's been able to.
I'm currently submitting for post doc, and hope to be able to work 0.4 FTE, but over 3 days so I can fit in with my older boys school time.
In common with many working mums, I possibly don't spend as much time at my desk as other colleagues, but my goodness am I focussed when I do. I tend to plan papers etc on the swings in the park, or while pram pushing.
Don't leave it too late to have babies. If you're motivated, when you get back to work you'll catch up. At this rate none of us will be retiring until we're 75 anyway.