Do I have to?

I always try to get the kids to go potty just before their parents pick them up so they don't have accidents in the car.

It is just about time for Cash's mom to arrive.
Me - "Cash, your mom will be here very soon. Let's go potty before she gets here."
Cash, reluctantly - "Okay." And he takes his sweet time meandering to the bathroom.

Cash sits on the toilet and successfully tinkles.
Cash, doubtful - "I have to do this?"

Me - "You went pee already. Good job. All done?"
Cash, as he climbs down - "You say I gotta do this? In the potty?"
Me - "Yes. Big girls and boys go pee and poo in the potty, not in a diaper like babies."

Cash raises his eyebrows - "Okay then Auntie. Mommy comes soon so I go pee for you."
Me - "Thank-you Cash."