Who's in the what now?
The book seems like it would function as wish fulfillment for a child listener. When mommy says, "It's time to get out of the tub, we have to go eat," maybe the kid wonders, "Why can't we eat in the tub? If I was the boss, we would eat in the tub. You'd listen to me if I was the king."
Message
Baths can be fun. Or, it would be awesome if you could do whatever you wanted to do, but eventually you probably should face up to reality.