Document everything. You do need to talk to an atty before you say anything to her. I recommend the forums over on forum.mensdivorce.com formerly known as Dadsdivorce.com . Especially look for "The List".
If you just simply file for divorce you will likely get totally hosed with not only child support but long term alimony. You need to document that you are not in agreement with her being a non-working stay-at-home-mom (SAHM). Generally the courts will assume you both agreed to whatever your situation is, which means they assume you should keep that arrangement after divorce. Thus, huge alimony paid by you!
Your lawyer can advise you on how things work where you live, as well as what kind of documentation is needed and legal. For example you may be able to legally record conversations with her where you express your disagreement with her not working, and her response of she doesn't feel like working.
This is probably a 3 month or so process you'll be in before actually filing any paperwork.
If you want to save this marriage, first you have to be ready to end it. But you had better be set up to end it in a way that doesn't hose you. Consider, too, that your financial well being after a divorce will directly affect your kids' well being.
You could try in the meantime something like taking her to a financial planning workshop. I really like Dave Ramsey's stuff. His Financial Peace University is a very inexpensive program run by regular folks in places like churches, civic groups, adult education, etc. Perhaps you can scare or motivate your wife into being more interested in the financial future.
I also suggest you don't blow this off. My wife made calculations she didn't need to save for retirement due to a nice inheritance coming her way. I saved like crazy. Then she hit me up for divorce and wanted half my retirement savings! She'll still be getting that nice inheritance, and probably well before her full retirement age, so her future is well funded. Meanwhile half my finances are gone.
And over 20 years marriage in this state means lifetime alimony.
You're much better off getting divorced sooner rather than later. And if your finances are tight, even better! You have less to hand over now and less to pay her long term.
You could also try talking to a marriage counselor either solo or with your wife. It would be a way to document the situation for court, too. Make sure you're the client, not your wife, so that you can waive confidentiality with the therapist if it should become desirable to get his/her testimony that your wife refuses to work while you want/need her to.
If you're in the USA, your employer should offer EAP, which is Employee Assistance Program. Free and totally confidential counseling. It would be an easy and free way to get started on couples therapy. Go yourself alone for the first session.