We got a Will drawn up after at least one of our children was born (I forget exactly how long it took us to get round to it).

We used Will Aid, where you can get a basic Will drawn up by a solicitor who has offered to do it for free, in return for you donating to the Will Aid charities. It looks like it's closed at the moment though, and "Will Aid will run again in November 2010" for those in the UK wishing to take advantage of the scheme.

We each left our entire estates to the other (I think!), but we had to consider the possibility we might both die at the same time (in a car accident, perhaps). We therefore had to think of who would look after the children, as well as what would happen to our assets.

One thing we were advised is not to put, "I leave my money to my (currently only) child X", but instead, "I leave my money to be divided equally between all such children as I may have at the time I die," as this means you don't have to alter it if and when more children come along.

We got our wills done in late 2007. Two different law firms here in Massachusetts stressed the importance of the following five documents, which we had drawn up.

1. Last Will and Testament: appoint guardians for your children, an executor of your will, how to devise and bequeath your estate (i.e. to your Family Trust)
2. Family Trust: detailed rules for the Trustee you appoint (not necessarily a guardian) about how to provide for the care, comfort, support, welfare, and health of your children (by distributing your estate, over time) in the event of the death of the Donors (i.e. you). Note that you must specify the trust as a beneficiary (after your spouse, probably) on your life insurance, retirement accounts, investments, and other assets.
3. Durable Power of Attorney: let someone (i.e. your spouse) make legal decisions on your behalf if you become incapable of making them yourself
4. Heath Care Proxy: appoint an agent (i.e. your spouse) who can make health care decisions on your behalf if you become incapable of making them yourself
5. Living Will: should they pull the plug on you??

I apologize if I misrepresent the purpose of these documents. I am not a lawyer, and your mileage may vary depending on where you live.

I had a great plan to get everything organized before I gave birth . . . and then I went into preterm labor ;) I think this question has a lot of good things to consider.

Here's what we've specifically been working on:

1. Life insurance. In my husband's case it's to provide enough financial support to get the kids to college if he were to die. In my case, it's to compensate for stuff I do around the house such as childcare and house cleaning (truthfully, we probably don't have enough to cover everything I do but it's a start).
2. Choosing guardians for your children. Several of the books I've seen recommend choosing both a caretaker and a guardian who looks after the financial interests of your children. We haven't gone this route as we're comfortable with the people we've chosen doing both aspects. We have chosen multiple guardian options with an order of preference specified so hopefully no one feels the child will be abandoned if they are at a point in their life where they can't really deal with suddenly having a child (maybe this is paranoia on my part but I like being prepared :) ).
3. Wills. This is the part where we've been slacking. It's on our list of things to do immediately but we're still not entirely sure how to go about finding a good lawyer as we don't know many people who live in our town so recommendations are problematic.