I highly recommend students to always start with a warm-up exercise. Imagine going for a run, but not doing the warm-up exercises needed to get your body ready to perform! 

Oscar Beringer's Daily Technical Studies include a wide variety of fingerwork exercises to strengthen our muscles that we seldom use. Students of all ages could use this book.

Me and My Piano is one of the books I would always recommend the younger students getting. It is a great first step to learning about the physical keyboard and slowly introducing them to playing both hands together on the piano.