I had a student, actually about the same age (11) with a similar learning difficulty. She had problems with distinguishing the clefs, and also left from right. She occassionally played passages in an opposite direction too -but more jumped back
Here’s a method I haven’t tried, but have heard about:
Turn the music on its side, so that the lines of the staff resemble the harp strings. (I think you want to go a quarter turn clockwise so that the low notes are to the left.)
I don’t have any harp students with that particular problem but I did just “inherit” a piano student who has trouble distinguishing between notes going up and down.