Call a local lawyer and tell him or her that you need a will. The lawyer will suggest that you get a living will, a healthcare proxy, and a durable power of attorney to go with it. Those are good ideas, too. For the whole package, expect to be charged between $500 and $1,000. If you can't afford it, you can create your own will using software available online. These are perfectly valid documents and will stand up in court.
Do it, say this, and sound smart: "I think it would be insane not to have a will and not to name guardians for the kids. Do you know what could happen if we die without one? A huge, ugly court battle over the money and the children. That's not how I want to be remembered."