Advice for having children during graduate school
Congratulations! First of all, you are in for a wonderful and life-changing experience.

However, your life is going to change quite drastically - so try to be prepared.

Some things that come to mind in no specific order (some of these are not specific to academia):


On the personal side, you must first realize that you will have much less time to work. Get used to the idea. The way to deal with this is to organize your time and stick to your schedules. No more random web browsing during work hours - use your time with your child as time for brain rest. As a scientist you may know that it is not always easy to be creative/focused on cue, but you will learn to get used to it with practice. 
As JeffE commented, if your advisors don't know yet, notify them. A supportive advisor can make a huge difference in terms of flexible-work time, working from home, and even moral support. I would also try to gently ask if they increase your salary or offer some other kind of financial benefits. Also, your graduation will most likely get delayed, hopefully not too much - try to see that your supervisors are ok with that.
If possible, have your family help as much as possible.
The first year with a child is a huge change and can be quite difficult, especially in your situation. Help each other and be understanding towards each other.
Your school's HR can give you information about benefits you could get for children (healthcare, day care).
You can find tons of used baby/child stuff (toys, clothes) for free or very cheap, because they are often useful only for a short time.