I contemplated the same idea a while back because I'm constantly printing out colouring sheets for kids and it seemed like an app people would use.
Ultimately I decided not to build it because there are so many free colouring page sites out there, and there's a non-zero environmental cost to asking Gemini to generate a happy bat image.
Pretty much any colouring page a kid could ask you for is already a Google search away. In fact the challenge is not finding a happy bat colouring page but narrowing it down from the thousands of results.
I just don't see the value in using Gemini to add yet more generic colouring pages to the millions that exist. I'm sorry, I don't mean to poop on your work, it's interesting to read the tech implementation but ultimately I don't like creating walled gardens with AI generated alternatives to what's already free