You might look at the series. PIANO DISCOVERIES by Janet Vogt & Leon Bates. Published by Heritage music press.
I'm switching two younger sibs for the same reasons as you stated previously. I will be using the Piano Discoveries (lesson book), the Theory Discoveries, and the Masterclass Series. All three coordinate together. I chose this method because I like how the pieces sound. I've been impressed with J. Vogt & M. Kelsey's compositions.
The books are appealing, lots of color and character. I'm quite excited to start using them!
Just click here Piano Discoveries
and you should be able to get to their website. However, it's not the best website. I recommend that you see the books for yourself. On their website there is a place to request a free book, I just received mine about a month ago.