I have 2 children. Both of them born after my diagnosis. They are now 17 & 19.
Your quality of life won't necessarily go down hill- not sure where you got that idea from
I did birthday parties, we have gone to Disneyland twice, I went to all of their school functions (still do), I ran a taxi service for my kids and their friends, took them to the movies more times than I can count, I was a soccor mom, indoor jungle gyms and so much more.
I have never had any regrets.
Having more than one child is going to be difficult even if your in perfect health. No one ever said raising a familiy would be easy.