Before I go through the essentials for any keyboard that is going to be used to learn piano on I need to correct a common myth that I heard so often as a piano teacher.
The myth that a child can learn piano on anything - even if it is only a toy piano or keyboard is a belief that has ruined many piano lessons.
This myth is a BIG problem for a piano teacher. It is actually no different than saying it is okay to learn to play soccer with a child's tiny hi-bounce ball. Hi-bounce balls, the tiny rubbery balls that have a massive bounce when you drop them on the floor, are great fun for kids. They can certainly help a child's coordination too but you wouldn't think of attending a soccer lesson with one. These small balls are just too different and they would end up not knowing how to play correctly with a normal sized soccer ball. It would be almost impossible for them to join a soccer team because they would not have learnt the right skills.
Learning piano is exactly the same. If you start with something that is vastly different to the real thing it will create a lot of problems with learning. I have had child students who came to me after learning on a toy (or poor quality) keyboard for a few months who just could not make the change to a piano or good quality keyboard. So parents beware when you choose an instrument. A keyboard is fine - but it must be the right type and never, ever a toy.