I’m trying to locate a book I read as a child but which don’t remember the title of. The story was about a child (a boy, I think) who needed or was challenged to jump over a high wall (maybe a castle wall). The wall seemed impossibly high to climb. However, there was a pile of boxes/crates nearby so the child used his/her imagination and began stacking the boxes one-by-one to effectively form a staircase that could be used to scale the wall.