There will NEVER be a good time to have children, I've heard this many times before, but I'm convinced there are bad times...or badder times to have them. I think that having children at a critical period where you really have to prove your capacity (such as during school) to land an ideal residency would be the least desirable time, career-wise.
As such, I plan to have children during residency...hopefully beginning year 2 when I'm settled in. Yes, residency is a time to learn but it seems to be treated more like on-the-job training than schooling, perse; you won't lose marks or negatively affect your GPA if you miss some time at work to take care of the family. A little older, a little wiser, a little wealthier, more established...residency seems like the least worst time to have the little bundles of joy.