Many of our words are the same, but you asked a question and I provided an interpreted answer.
Edit: To say it specifically
The fourth character was drawn as a character representing an enlightened perspective. Rather than having multiple panels or having text to describe this, the character was drawn as a child as a shorthand since children come with that association in most cultures.
This is opposed to the other characters drawn in different walks of life's who have their own unenlightened motives which we assume by default within the context of the forth panel.
The issue is that you called it dumb because you interpret it as "only children" are enlightened enough to see the tree as a tree.
But I don't believe that is the intent of the comic, instead, they simply drew a child as a shorthand representation for the concept of enlightenment.
I believe any person can be so enlightened to see something as it is, and not what it could be made into if they wanted to.
Therefore, I don't think the comic is dumb as you stated. I think the comic is attempting to motivate people to see things as they are and be enlightened.
Also there was a little humor in the misspelling of a common word when calling something dumb, in the way of "kettle calling the pot black"
Something about looking and not seeing. Or those who have eyes to see...I wonder if this is our mythical third eye, the ability to imagine, extrapolate eg child/inner child, in this instance.
The child is someone who doesn't have a financial stake in what the tree is used for. It's narrative shorthand for innocence, but it doesn't mean "only children think this way".
Otherwise it wouldn't be encouraging the readers, presumably adults, to think that way, because it would be impossible.