How do we arrange for a wheelchair to take us to our departure flight?

You can rent one from the kiosk, by the front door.

What you do is go to the airline counter, and explain to them that you need assistance getting to the gate, and getting from your eventual arrival gate to the baggage claim area. Allow extra time. However, airport/airline personnel will get you to the gate, (and baggage claim) by wheelchair or small vehicle. No need to rent anything.