As teachers, we often choose to follow a specific book series with our students. Doe example, the Hal Leonard Piano Lessons book series or the John Thompson Easy Piano series with younger or beginner students are very common choices. Each series has been carefully designed so that the basics are covered well and the lessons gradually get more challenging.
It is really about your personal preference as to which series to go for. Some of the books get harder quicker and younger learner might find this frustrating. In my experience, younger learners like learning new songs on a weekly basis and having several songs (such as nursery rhymes) more or less at the same level will help them keep motivated whilst building their song repertoire.
Personally, I find switching from one series to another most helpful and supportive for students. For example, John Thompson Easy Piano Couse Book 1 is a nice place to start, but Book 2 is considerably harder and so this is where I like to use Hal Leonard Piano Lessons Book 2 as an easier book to move onto next.
It is crucial to keep teaching the students about how to read music and the basics of music theory as well. For this, I find it helpful to have a specific music theory book (such as Music Theory for Young Children Book 1) as a supplementary book. This mentioned book has short exercises in it that the students can complete in their own time and so will enforce their learning.
Overall, I find that mixing it up and taking the most helpful bits from each book is the best way forward with most students!