On the surface, this film has a lot going for it. An interesting crossover premise, a compelling protagonist, some intense themes, and a heart-warming message. Unfortunately, it falls flat on all of these things.
Making Thumbelina a character who is easily abused and exploited is a refreshing take, but this was only really portrayed in the first 20 minutes and then turns into a basic kids' adventure film with conflict that's often easily overcome in like 5 minutes. There's also a lot of inconsistencies and plot holes in the story, like how Tom Thumb was originally a slave of the circus but has somehow escaped with no explanation. Thumbelina, being as headstrong as she is, never thinks to run away until another character tells her about her village.
Out of all the characters, Thumbelina is the only one with a clear arc and personality. Unfortunately, she's often inconsistent and doesn't really go through any obstacles that test her flaws and grow her as a character. She's also pretty unlikeable as she insults almost every character that she encounters on the first meeting.
On a technical level, the colorful and expressive animation will appeal to children, but there are some points where the animation looks sloppy and unfinished. The music sounds nice, but it ranges from bland (I'm All Alone) to bizarre (One Two Cha Cha Cha).
It may appeal to really young children, but some of the dark scenes might scare them or make them uncomfortable. But I think this will be too silly and safe for the preteen crowd. Personally, I think it should've just went the dark route as that's where the movie really shines while still keeping a fairly lighthearted atmosphere for the younger children.