A startup company is basically a newborn baby. It requires continuous attention and care, cleaning and feeding. Except, unlike in the case of twins, it can't be given a bath and fed alongside another baby. This baby is an attention-hungry baby with special needs and demands, which requires focused, dedicated care to thrive and grow.
And to reverse the analogy, caring for children is in itself a full-time job, just as hard if not harder than starting a company. When shit hits the fan and you get 3 hours of sleep every night for a week, which of your dependents becomes your priority? Are you going to care for your children or get hard work done?
I'm not arguing that it can't be done, but I also think there's little argument that taking on both significantlty drives down the odds of success, to the point that it's questionable why any VC would give them their money.
I say either found your company before you have kids and wait until the company is no longer a newborn, or have your kids before you found and wait until the kids are no longer newborns.