ABANDONED. YA Fantasy Children's Book With a Child Switching Scheme? Written in the 60's-80's (Spoilers Within)

[Marjorie]

I remember the plot of the story involved a young girl who wasn't a very good witch (or a witch who was treated unfairly), and a child-switching/changeling theme (I was inspired by watching Good Omens on Amazon). I believe the child in question (let's call him Bob, not his name in the book) was supposed to be a prince of the fairies/fairy kingdom, an orphan given to a family that already had a child (let's call him Charles). The family Bob is brought up in treats him like garbage and spoils Charles, leading him to become a fat jerk (unfortunate but part of the plot for some reason?). Charles is eventually kidnapped, under the pretense that he is the true king, meanwhile Bob and witchy girl become friends some how? I remember it also had some simple pen and ink illustrations in between chapters, but I could be wrong. I read this book between I think 2006 and 2010 but it's a bit older I think (speculating between the 60's and the 80's). It is not Good Omens or Harry Potter! Just has some similar elements, may be British but can't remember.

[Summer]

I think this is The Secret of Platform 13.

[Kris]

No response, moving to Abandoned folder.

Marjorie (OP) was last active on the site in February 2020.