1. Imu has tiny hands and he is small compared to a hat. Imu being a child would explain this.
2. Imu is small compared to the throne. The throne being made for giants doesn't make any sense:
a) because the World Government would never allow a giant to be their ruler due to racism.
b) It would still be too small for a giant.
Imu being a child would make perfect sense to explain why the throne is too big for him.
3. Imu easily gets angry, just like a child. Yes, there are adult characters that are also impulsive, but this type of behavior would fit a child.
4. Oda loves foreshadowing. What if Sugar is foreshadowing Imu? A forever young child with a crown on her head who rules unfortunate individuals...
5. Imu's speech pattern in Japanese sounds very childish. More informations here
https://thelibraryofohara.com/2023/06/04/chapter-secrets-chapter-1085-in-depth-analysis/
6. Possible Akira reference
https://www.reddit.com/r/OnePiece/comments/13hi2jz
7. Possible reference to Where Wild Things Are
8. This scene from Doflamingo when he was a child