I think you all are missing some general point.
First being as others have pointed out, this is clearly a parody or reference to another series scene.
Second this is manga/anime. The medium intrinsically exaggerates everything for effect. Some are more exaggerated than others(cough cough Japanese food basically being orgasmic food of the gods). And some can be far more grounded in realism. But I do not know of any series of the medium that does not do it in some capacity. If you want to look at it realistically, you need some sort of lens to overlook/translate into its real equivalent.
Third, is the language culture barrier. How people convey and express thoughts varies in cultures differs. It is entirely possible for us to take the reaction to the performance way more literally than it is meant. It may be simply meant to convey being impressed with how well the "tree" blends into the background and behaves like the real thing(ie is solid and does nothing). Or that it gives the impression/feel like a real tree, or real tree prop. Leading into the next point which brings up a equivalent.
Fourth is the type of performance here. Being a good tree could potentially be a difficult performance.
For example take living statue performers, where people imitate being statues, to the point of convincing people they are a statue.
An example of this being used to great effect to prank/surprise people.
Now at first glance you may say this is apples and oranges. Just standing still in a tree costume doing nothing is not that hard. Sure. It's certainly less difficult from realistically portraying emotions or memorizing and reciting lines.
But have you met a real kid around the age of 5 before? There is no chance in hell a kid that age would not be swaying, twisting, moving around, picking their nose, stretching an itch, leaning over, mumbling, getting drowsy or doing something within 10 minutes or less. If I saw a kid that age simply stand in one place and keep their arms up holding the branches I would be extremely impressed.
And a kid who could actually be so motionless you question whether it is a prop or person(see living statue comparison). That would be absolutely incredible, almost unthinkable unless that kid was trained specifically for quite a long time, AND had the temperament to actually do something so boring.
None of this changes the absurdities of this manga. "Oh hey you stood motionless for a while. I better scout you as a genius child actor" is dumb as hell. Same with magical charisma powers. Just because we can speculate and rationalize some of this doesn't remotely mean it was written or meant that way or is valid in this instance.
It just means that the basic plot can make sense, the baby doesn't have to be thrown out with the bathwater. A scout is there watching a kids play. He notices a kid who is doing a perfect job playing a tree, completely unlike her age. The kid also remains completely calm and continues to do the role when everyone else is panicking, including some adults. While continuing the role she even helps calm her classmates a bit. She piques his interest. So he asks the kid some questions after the play, and is further interested in how calm and mature she is for that age. With a great attitude/demeanor. He thinks a kid like that might have potential to be trained as an actor, and might have some interest.(a mature calm kid with some charisma among her peers, who commits to her role would likely be able to follow directions from directors well)
Anyways, the point of series like this is not to seriously engage the brain.
It's been fun wasting time pointlessly finding rationale for this chapter. Especially watching some of the performances of living statues. I got serious respect for their capabilities, I would be twitching and swaying all over the place.