Rating: WARTY!
The short answer to What Happened on Planet Kid is: Not a darned thing! Admittedly I skimmed this audio book because it plumbed new depths in the bottomless mine of the ineffably boring, but Planet Kid was barely mentioned. The two kids were almost never there and pretty much everything that happened in this novel happened elsewhere. Not that it was actually a planet, of course; it was simply a hide-away that they had created to avoid boys, but THEY WERE NEVER THERE! Nothing happened. The novel's title was a complete lie!
Not only that, but reader Kate Forbes not only sounded too old for the character, she sounded like Clarice Starling from the movie Silence of the Lambs! I honestly couldn't take it seriously, but it wasn't funny, either. If you consider a twelve-year old's "Dear Diary" from half a century ago, with all the interesting bits cut out, entertaining and engrossing, then this is for you, otherwise you might want to give it a fly by - I planet kid you not.