Just adding a few suggestions:

1.  Regulated by the AGO's office in many states and state filings may be required (you might check there first to see what the state's requirements are - see below).

2.  Some states allow formation as corporation, or as a trust, but they will require organization documents to match (which means needing legal counsel to draft same) since nonprofits require unique language in their organization documents.

3.  If state's filing requirements are nominal (e.g., they look only to the Federal filings having been completed) then perhaps only a small fee each year to the AGO's office, if any, for solicitation license.  Otherwise, some states require a full state filing (e.g., Form 990 or 990-EZ) even when the IRS only requires a Form 990-N postcard. [Not a good combination as you end up doing a 990 just to attach to the state filing.]

4.  Don't underestimate the amount of work (and parties) involved in filing a Form 1023 as it is not a simple process.  [I have not completed a Form 1023-EZ so I really can't speak to that one.  It may be easier.]

5.  Probably the biggest issue you are going to face is who's going to take over when the kids graduate from college and disperse in two year's time?  Are you going to become the director, trustee, officer and accountant for the organization going forward?  Liquidation usually requires a request and the approval from the AGO's office/court to withdraw (or merge) since the assets, if any, must go to another nonprofit (i.e., no private inurement of benefits) and counsel will likely be required again.

Not to discourage the kids from the process, but the suggestion above was a good one.  See if the girls can host one or two annual fund-raisers through another, existing nonprofit (i.e., where the donations will be deductible, and the wine & cheese gala/auction will be fun for all).  Often times the hospital can facilitate (or has a nonprofit entity that can facilitate) such a process or event for them.  It removes the administrative headaches.  Just from experience, the MDA used to do exactly that for a group of friends who ran an annual fundraiser/auction for them hosted at a nice restaurant each year.  Fundraise Your Way for MDA | Muscular Dystrophy Association.  I'm thinking the hospital might do the same if it has such programs.  Hope it helps.