why is Zen-Oh portrayed as child?
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