How to choose the right size keyboard or digital piano.
What Size Keyboard? This one is really important, regardless of your reasons for learning to play.
A full sized keyboard has 88 keys. And I believe that if you buy a full sized keyboard, you are going to be able to play every musical genre without a problem.
More advanced students will almost certainly run into difficulty if you intend to play classical music on a smaller sized keyboard.
And if you are buying with the intention of studying classical music, you will want to be able to use it to practice for practical or performance exams.
If your keyboard is not full sized with touch responsive keys, this will not set you (or your child) up to do well when you go into an exam situation on a real piano.
However, having said all of this, if your young child is a beginner, you are far better off buying a five octave keyboard from a reputable brand range, and then trading up to a bigger keyboard as they progress in their learning.
It is far more important that their keyboard has a good sound, and is touch responsive when they are starting out. Otherwise they simply won't enjoy playing.