Thank you to everyone who offered tips on how to keep the little girl's pants up. I did some internet research and found a belt I wanted to buy. Then I came across an even better belt, one I thought I could replicate. So with items I already had on hand, Velcro and grosgrain ribbon, I fashioned Callie's new belt. It is working wonderfully. It holds her pants up and doesn't require her to unfasten anything before using the potty. The only down side is the pants have to have belt loops.
And, just for the fun of it, Callie is wearing a her first piece of 3T clothing - the shirt. It might be a little big still but she is well over 3, and I was tired of 18 and 24 month clothes.