why is Zen-Oh portrayed as child? What I want to know is the writers reasons.
Does he say why he did it?
I would assume he wanted the innocence of child but the moodiness and lack of any logical behavior doesn't go well with all that power.
I think it's mainly to incorporate humor into the plot. This is not the first time we have seen characters like this. We've seen Majin Buu back in the Buu saga who acted like a child despite being insanely powerful. Toriyama is known for creating characters of this manner. The fact that we get to see the strongest beings in the universe, the gods of destruction be terrified of these kids, incorporates a lot of humor into the storyline