It's at least as funny as "why did the chicken cross the road," which is only a joke inasmuch the punchline is merely a statement of the obvious in the framing of a joke (the surprise is that the punchline sucks -- making it a groaner). I submit that that chicken/road joke wouldn't stick around if it wasn't funny. So, this joke stands on the shoulders of the chicken/road joke, making the obviousness that much funnier within the shared cultural context. Moreover, it adds a layer of absurdity (imagine the literal sun climbing a tree) with a linguistic confusion (aka pun) as we do refer to the sun "climbing" the sky. And finally: for some reason, our culture is more tolerant of groaners from "dads," so much so that some call them "dad jokes." Your child has inverted age and gender norms with this joke, making it so incredibly funny that you are blinded to the truth: this is comedy gold. Watch that kid, she's going somewhere. It might be an open mic night at a skeezy comedy club.