Both of my kids started learning the harp at 5 and they use Bonnie Goodrich’s Small Tunes for Young Harpists, Betty Paret’s First Harp Book, and Phyllis Schlomovitz’s Beginner’s Harp Book. All of these have good pieces for technique building. I actually started with the First Harp Book myself, LOL! I think Small Tunes for Young Harpists is the easiest of the three. There are definitely some challenging pieces towards the end of First Harp Book and Beginner’s Harp Book. Hope that’s helpful. 🙂
I think Small Tunes for Young Harpists by Bonnie Goodrich is a good choice, maybe also Basic Harp for Beginners by Laurie Riley? I haven’t looked through the whole thing but it seems to take things slow and simple. I’m learning as an adult, and I like Old Tunes for New Harpists by Mildred Dilling. It’s a fun one I’ve been working on with my harp teacher, and I think it could also be used with kids, lots of classics and it progressively gets more challenging.