88 keys: While you can get by with a 76 key piano, you would not want to get a 61 key keyboard since these are generally sampling instruments or toys. We recommend that all of our students have 88 keys to allow them to become accustomed to the size and spacial feel of the standard 88 key piano.
Fully Weighted keys: Watch out for Clavinova or YDP series with an "S' at the end of the model number. These are usually semi-weighted keys that do not offer the same experience as a traditional piano. Many people with semi-weighted key pianos think they have a fully weighted key piano, but there is a substantial difference. Touch sensitivity is very important, but nowadays, just about every digital piano with fully weighted keys has many levels of touch sensitivity to replicate the touch range similar to a real piano, so this is no longer an issue when selecting a piano.
A Real Pedal: Ideally, you want a two or three pedal board that is built-in to the digital piano, but really, only one pedal is needed for beginners, though we insist that a real pedal is used rather than a plastic "sewing machine" type pedal. These plastic pedals tend to slide around when playing and take away from the experience. You can have a free standing pedal, but it should be metal and operate like a lever.
Other than these three essential items, you will want a digital piano newer than the late 90's because by then, just about all digital pianos changed from wave sounds to stereo sampled piano sounds which are still used today. Some pianos, like the late model Kawai's, use real wood keys which shrinks the differences between the digital piano and the acoustic piano as far as the touch part of the experience goes. We particularly like the Kawai, Roland, Clavinova, and Privia series. The Casio Privia PX-130 is a favorite for our students and customers when it comes to getting an affordable rental piano that meets all the requirements for proper learning. Once your child advances, you will want to consider upgrading to a higher end digital piano, or an acoustic piano. Feel free to email us if you have a particular model that you are curious about.