I have just completed a masters with four children. Youngest two are twins (5 months) which I had once I'd finished all the major work and submitted my thesis.
I had first child during BA degree, second child when complete and then started work full-time in crap, low paid job at the council. Your employability with graduate employers often depends on how close you live to major cities, and how flexible you are with where you work. i.e if you are able to re-locate, and frequently. Graduate jobs near me are very rare, and I feel I am already at a disadvantage because of my family commitments.
i quit said crap job, and went to do a masters, which was paid for and in another town (so i wasn't on campus very much as you can imagine). Then I had twins. Anyway, my options are now severley limited, mainly because of all the time I have spent out of career, and bearing children has meant we are fully reliant on my husbands secure job and have no chance of moving anywhere else. My job opportunities are severly limited. I currently do not work and in all honesty, have stopped looking. When I had just the two children, working full-time was just about possible logistically, but barely worth it financially. With four, for now at least, working is nigh on impossible. This might change as they get older, so take a long term view of it all.
It's a balance - the more kids you have the longer you are putting off your return to the workforce, and the less attractive you will be to employers (even if they can't admit legally that having family commitments puts them off). So some people would just chuck it for a bad job and have more kids, I know I have. I will concentrate on them for now and look for a job later. I have lived to work, so it's taken me a long time to be at peace with my decision, but really I've found you can't have a big family and a good career, even with a good degree (I've got a first class, and a masters), and even if people are chucking offers at you.
As for you - I would look into career options seriously before I made any decisions. Then if you find that work is limited at the moment, it might be a good time to have more children, though I feel that if you really wanted more kids, you'd just do it unquestionably.