What documents do we need and is it one we can do and file ourselves?

You would do this through a will, with each of you designating who would become the guardian or guardians of your children should you both decease. You should also consider who would be the conservator, that is the person who would administer the children's money. This could be the same person as the guardian, or could be someone else.

You should have will or trusts, and statements of intention and nominations of guardian/conservators for your children in the event of your death.

This should be in a will. You really need to spend a little time and money to talk with an estate planning attorney because there are more issues than just who takes care of the kids. What about the proceeds from your life insurance - do you want the kids to get it when they don't know what to do with the money? I usually suggest that there be a guardian for the kids and a trustee for the money. That way if one or the other goes off the rails, there is somebody to step up and ask a court to take care of the issue. Also, you need to think about durable powers of attorney for financial and childcare issues. If you are in an accident together and are both comatose, somebody needs to be able to step in and take over until one of you are capable of going on. And don't get me started about living wills and a medical power of attorney.

What you're speaking of is, believe it or not, a will; you can identify who you wish to be your children's guardians in the event you both die.