Why does he talk to an adult women like he talks to a little child. I know that the trick is for little children, but the assistant on stage was an adult. So he could have talked to her normally.
And as for that matter, why do people don't speak to children in a normal voice? Try the following. Don't change your pitch or speed. Talk to children they way you talk to other people. Over exaggerated emotional speaking voice is okay for children under the age of three as they cannot differentiate between the tiny changes that we put into our words to convey emotions. But once they have a passive vocabulary they are and should be able to understand when they are spoken to.
Most of the time speaking to children in that way seems to be laziness on the part of the adult. The adult wants to make sure the child understands that when you say "Kevin, get the ball!" it is not misinterpreted as a mean thing. They want to make sure the child understands that this is a positive thing.
But think about it you can use your normal voice and make sure the child still understands your emotional value by adding content. Instead of saying "Kevin get the ball!" in a overly friendly voice say "Kevin, would you please get the ball for me! Thank You" in a normal voice. A child older than three years will understand both: The command and the emotion that is conveyed.
Robert Baxt seems to have this problem in general:
I don't even know if he uses his real voice at all. But his performing voice is unnatural. Matt Hollywood, Shawn Farquhar and quite a few others seem to suffer from the same sort of problem.
So use your normal voice! The voice you use when you talk to your wife. And if there is emotion to be delivered, try doing it via words, not via higher pitch.