Honestly, I'd have kids now/sooner. Kids are dependent on a biological clock, whereas you can pay off your mortgage when you are 30, 40, 50, 80, or even 100. There is no time limit to paying off a mortgage like there is to growing a human inside you. Plus, the sooner you begin starting your family, the more time and better chances you'll have (whether you want one child or more).
My DH and I are only 26, and originally we were thinking of starting our family when I was 31 or 32 and within ~$200k of FIREing. However after casually mentioning this plan to my mom, she told me that I have family history on both sides of infertility and unexplained miscarriages. It took my mom 4 years to get pregnant with my brother. After that, knowing that I would probably have problems getting pregnant, DH and I decided to instead start trying for a baby sooner, right when I turn 28 or so (so 4 years sooner than we originally planned), to give us the best chances of having the family we ultimately want. This meant pretty much scrapping our original FI plan -- now we will get to ~50% FI before hopefully getting pregnant and then let our investments slowly grow to our FIRE number. We are entirely happy with this new plan, even if it means a few extra years working.
You can also compromise and get a bit closer to paying off the mortgage before attempting IVF. It isn't either get pregnant now or at 31 -- why not 30?
I second the idea to refinance the mortgage to a longer term. You can always pay extra when you have extra cash, but if you have a high min payment it will be stressful. Also there are many MMMers around in the "dont pay off your mortgage" club who advocate paying the min on your mortgage over as long a time period as you can get as long as you are able to take advantage of the historically low interest rates. I'd look into that, because not all debt is bad.
Also, MMM is about life optimization, not just solely financial optimization. Just imagine the "worst case scenario" for both children and mortgage/finances. Youre looking at potentially being completely unable to have any biological children vs paying off your mortgage what, 5-10 years later than you planned? Working for ~5-10 extra years when your kids are probably in school anyway? Which is worse to you? Then what can you do to mitigate that worst, worst case scenario?