I teach beginner piano lessons and I've used both Bastien and Alfred's series and both are good. I recently began teaching some younger students with Little Mozarts by Alfreds and it teaches piano and music theory through a story about Mozart Mouse and Beethoven Bear. This might be too elementary for your children though. The newest books I just began using with my 5,6,& 7 year old are the Faber series: My First Piano Adventures for Beginners and it's great very colorful and has cool characters that teach different elements of music. Hope that's not too much info. Your best option is to go look at a music store that sells piano lesson books to give you a chance to look through them yourself and see what appeals to you and would fit your kids. Just like school curriculum some fit you better than others :)