You could go back to the Stan Lee & Jack Kirby original run of the Fantastic Four that started it all. His birth takes place in Fantastic Four annual #6. The best way to read those older comics, unless your local library has the books in their collected editions, is to sign up for
Marvel Unlimited. You can get the 7 day free trial and go on a binge reading stuff. I know a lot of younger fans really don't like the old style comics but I find you get a lot of background info that way. And you can just skip over stuff that doesn't look interesting to you.
Sue and Reed also have a second child Valeria and her history can be quite confusing because it was originally tied to a trip the Fantastic Four took into the Negative Zone in issue #254.