Most US airports that don't have jetways either set up a (long) ramp with switchbacks or have a lift to get wheelchair users to the airplane door. If neither of those options is available (unusual), the passenger will be transferred to an aisle chair at the bottom of the steps and carried up by 2-3 staff.

The wheelchair user and her family should familiarize themselves with the Air Carrier Access Act, which regulates access in air travel - take a look at the stickies in this forum for links to the ACAA and other resources. You don't say how old the daughter is or how long it has been since she has traveled by air, but all of the mainline airlines have improved tremendously in the last 10-15 years in their familiarity with and service for wheelchair passengers.