A bit flummoxed

Here I am, in my attic-cum-study, hoping the roof will hold and the bucket won't overflow, and finding myself not quite mistress in my own home, what with the water sneaking in through loose tiles and my new story showing me who's boss (not me).
I had this silly idea for a picturebook about a nasty kid who ends up (or thinks he is going to end up) in a 'boy sandwich' at the hands of his two aunts who may or may not be witches.
It seemed pretty straightforward so I picked that idea out of the project box and started working on it, i.e I opened a brand new Word document, saved it as 'Boy Sandwich' and started typing. And then stopped. And started again, with a different opening. After I did that three or four times, I declared to my typing self: ''There's something wrong''.
I thought about it for a while and realised it wasn't the writing that was causing trouble; it wasn't finding the right words to launch the story that was proving tricky. It wasn't even the story itself. It was a) the main character (evil boy) and b) the format (picturebook).
I've written a few nasty-character stories in the past and imposed some terrible fate on them or given them a glorious ending. But for some reason, last week, I found that the boy in the Boy Sandwich project refused to be treated as a nuisance. That he demanded to be painted as a human being with light and shade, good points and bad points. The works.
As a result, the size and shape of a picturebook didn't work anymore, as my Boy needed more space to plead his case and develop. I'm now 1700 words into the story and Boy has yet to meet the Aunts, and I can already see places where I'll have to go back and add bits so that it all makes sense and isn't rushed.
I've no idea where this is going (as my original ending worked really well for a picturebook but might look a bit slim and 'babyish' in the new order of things) or how long it'll take us (me and the boy) (or rather the boy and me) to get there. Even the whole sandwich business may have to go out the window... and perhaps be recycled in some later project!
Exciting times...

(The flummoxed rooster, wondering in actual fact how he can get rid of his hiccups, is from La véritable histoire du coooq Figaro, written by yours truly and illustrated by Dorothée Jost)