Unidentified Children's Movie, Early-Mid 2000s

I'm asking for a friend who says he has been looking for this for years, apparently since they were n the 4th grade. Here's the exact quote:

"It was a movie, probably direct to DVD but it may have had a theatrical release, I’m bad at keeping track of these things. I first saw it in class, (4th or 5th grade so this would be between 2009-2011) just something our teacher put on because it was probably close to a holiday or something. The whole film had a sorta low budget live action kids film bridge to terabithia vibe. The kind of thing you’d see played Cartoon Network. It centered in these two kids, a brother and sister if I recall. Maybe they were just friends or something but I know it was a boy and a girl. The whole beginning and middle are a blur to me but I remember them being lost in some sort of pocket dimension middle hook building. Basically there trapped in this weird alternate reality zone, under the control of this monster thing. The monster appeared to be this sort of large black mass. Even as a kid o remember thinking the CGI was kinda weak, but it was this large goopy black mass with huge tendrils. Think the flood from halo but smooth and black. At some point it either creates or somehow possesses a small young boy. The protagonists communicate with the monster by using this boy as a sort of middleman. Again a lot of the plot details are blurry but I remember this being one of the first films I saw with a fakeout ending. Essentially the kids think they’ve escaped from the monster and are back home. Their mom is making dinner and everything seems idillic and fine. Except the kids slowly realize that something is off, and eventually figure out that they’re still trapped and the monster yanks them out of this false reality. I don’t remember what happens after that or how the movie ends. The last scene I can recall clearly is the two kids in this big like… liminal space indoor swimming center type area. Basically a big room covered in ceramic tile. They’re talking to the monster and one of them says something to the effect of “is this your brain”. That could be a totally misremembered quote, as this was like 10 years ago, and my memory has never been great. Anyway this movie really left an impact on me as I still remember it all these years later. Please help me end this search!"

Thanks in advance for any help guys!