I've got both, so I guess it makes me a subject matter expert.
There will be times, often quite regularly, that a business will keep you up at night and/or wake you early in the morning with thoughts of things left undone or general stress and anxiety. The difference is with a business, you can probably force yourself to fall back asleep. With a wailing baby in your arms, it's their choice to submit, not yours.
Just when you think you have a business skill mastered (e.g. sales, project management, accounting) something new will come along and flip you on your head. Anyone who is a parent knows this is a harsh truth; your child is perfectly safe/content on a tricycle, but now, suddenly, they want to ride a two-wheeler.
Even in the happiest, most joyful moments (e.g. signing a big contract, hiring the first employee, moving into your own office) it's only a matter of time until you're back down in the dumps again, frustrated, scared and confused. Kids have a knack of doing this to you on a regular basis; curious, risky little humans that they are.
You can never really get ahead in business: there's always more bills to pay, work left unfinished or to-do list items that await. Same with kids: there's always laundry to do, books to read, bedtime to make, meals to prepare or lessons and parties to attend.
If you don't keep on top of managing your business image through careful marketing and communications, somebody else will do it for you. In the case of kids, if you aren't careful about what they say or what they learn, somebody else will take on the task without you being aware; that is until they surprise you at the dinner table with some choice new words or phrases.
Running a business and raising kids are some of the most challenging and rewarding experiences a person can thrust upon themselves. That said, I'd argue raising kids is more rewarding in the long-run; a business can't draw wonderful pictures or say they love you when you need it most.
Writing your own paycheque is great, but honestly, when a child wraps every inch of their little arms around you, you'll quickly realize that a child's hug is much, much better.