Ooh, ooh, I think I know the "dumbwaiter" detail! Except it's not a dumbwaiter, exactly. You know how someone said that the slam book scene is from Otherwise Known as Sheila the Great by Judy Blume? Well, so is this scene. This is where the girls are going to play hide and seek at Mouse's house, only Mouse's mother isn't home and Mouse doesn't have a key, so they sneak in through this little door that's supposed to be for the milkman to leave the milk. Or something like that. Obviously it's pretty small, and the fat girl (Sonia? Sonya?) gets stuck. The really silly part is that other people had already gone through ahead of her and could have just unlocked the regular door from the inside, but didn't.

This isn't the book with Jeremy and the dialysis, though, and I also agree with you that there aren't any BSC books where there's a friend with kidney failure.