The beauty of “And then I woke up” comes to every nine year old child at the same time I think. It is a combination of the following factors:
a) Some awareness of classic KidZor Lit which uses this trope (Hello Alice In No-Fun-Derland and WizovOZ)
b) Over ambition in writing a story for class (= this book writing lark looks easy)
c) Having had a dream that ends suddenly before things get good
d) Running out of time on the over ambitious story.

Thus we get classics like:

“Just then me and Steven were caught by the pirates. Steven struggled, but I got away, to look back. Just when I was going to rescue Steven with the knife I had rescued from the old pirate with the grey beard, they dragged him off to be eaten by canibles. AND THEN I WOKE UP.”

Often timed to finish with a side of paper, “And then I woke up”.

Unfortunately this is invariably seen as a cop-out ending (often as it is correctly identified as being because of b) and d) above). However tell that the Lewis Carroll, Jamie and the Magic Tourch and Mr Benn. Kidlit/TV is full of “And Then I Woke Ups”, because its instant restoration of status quo is actual very reassuring. It also allows the rest of the story to go massively off the rails, thus tempting the reader into the fantastic, and to push the limits of standard narrative.

Put it like this: it is a much, much better ending that “And then I didn’t wake up”.