I want the visual approach of tiny vs ginormous.

Why monsters?
Because they tend to be synonymous with 'nightmares' or 'scary.'
The term 'monster' is used to describe a vague variety of strange, fictitious creatures created from myths and stories. More often than not, a child were claim to have a 'monster under their bed'. This is the cause of stories told to the child, to scare them into not doing something or stemming from imagination, and would usually evolve into nightmares. In this sense they are being 'bullied by the monsters' as they are helpless against it. Thus, the parent will read the child to sleep or comfort the child. The parent is typically seen as the savior which has 'stopped the monsters'. 

Placing something tiny amongst these intimidating creatures creates a nice juxtaposition and transforms the tiny being as almost 'weak-looking' because of the size difference.
Of course, in the famous cliche the tiny defeats the big and is seen as brave in the end. The example of fighting the 'Big Bad' is most commonly used to convey the likes of good and evil, but I've used monsters as a story-telling device to visually display 'conquering a fear'.

I wanted to tell the story of overcoming obstacles (e.g the parent monster competing against all the bigger monsters) and the struggle of a parent wanting to restore hope in their child.
However, the idea of revolving around a world of monsters is not fixed and can be altered.