Charlie, not wanting to have to take a bath alone, kept trying to talk Frankie joining him. He tried all sorts of persuasion and begging, hyping up the awesome bath toys, etc.

Frankie stubbornly refused again and again.

Meanwhile, Elena kept trying to help by trying to force Charlie to go take a bath alone.

I finally got the chance to intervene and, pulling Charlie aside, I briefly explained reverse psychology to him.

He walked to the bathroom saying in an excited voice, "Frankie! I'm going to take a bath with No One!!"

The effect was immediate.  Frankie dropped what he was doing and ran after Charlie saying,  "I want to come, too!!!"

I just hope Charlie remembers this next time he really does want a bath alone. It's harder to switch Frankie's mind in the opposite direction.