My six year old fell in love with Malory Towers, listening on repeat to First Term at Malory Towers then diving into the full pack over the school summer holiday.
She would regularly tell us stories about each character in detail, and it is one of the few audiobooks that I haven't minded listening to along with her on repeat (and repeat, and repeat). She is very sensitive to anything remotely scary (including music or description, e.g. the Highway Rat's yellow teeth (!)), but loved Malory Towers from start to finish. Her vocabulary noticeably improved (along with some quaint Enid Blyton sayings!) whilst listening.