To get started one does not need to invest in a full 88 key piano with weighted keys. In fact, a $150-$200 portable keyboard with 61 to 73 keys is all that is required for the first 3-4 years of study so long as it is touch sensitive. That means that the harder you push the key, the louder the note will sound. Children can start as early as 5 or 6 provided they can comprehend everything involved in reading notes and understanding how rhythm works.
One of the biggest myths believed about learning piano is that if you can play classical you can play anything. I have found in working in band environments with classically trained pianists that nothing can be further from the truth. Many could play nothing but classical. This is because classical and popular music are approached very differently.
As the vast majority of my students (actually, all) want to play their favourite hits on Youtube or with their church praise & worship team and have no intention of playing with a symphony orchestra, I use a curriculum that helps them to understand the theory behind how popular music is put together. This gives them the freedom in a very short period of time to begin improvising using only a lyric sheet with chords.