Introducing music to 4-6 year olds
Remember, the following information is general in nature - expect to tweak and tailor the approach for each individual. Here we'll focus on 4-6 year olds.
The initial learning experience for this age should focus on cultivating a joy for the experience of music. One big point - The manner in which the content is delivered as important as the content itself.
This is true of any age, but extra crucial for young children who may not yet have the maturity to be as patient with the learning process.
I would recommend a very tactile experience in the first few introductory lessons which include:
1) Opening up the piano case and seeing the the parts and mechanisms that create, shape and craft the sound (children love this because of how sensory this experience is)
2) Basic keyboard geography - Exploring the recognition of black note groups (2 and 3) and what happens to the sound as you go to the right and left of the keyboard.
3) Learning finger numbers by tracing the outline of both hands onto a sheet of paper and sticking it up near the piano for the first few months. Young children understand concepts when they create their own learning guide.
4) Encouraging (but never forcing) an effective curved hand position by using a tennis ball/knee to naturally shape the hand and fingers.
5) Basic finger independence exercises (such as I love sticky candy floss) that incorporate singing to develop aural recognition of relative pitch, and musical phrasing* from an early age
* One really important tip that I would put forward, is to not underestimate a young child's ability to understand more complex musical devices such as voicing and phrasing. While a young child may not always be able to articulate verbally an understanding of such devices, they can often understand it at an intuitive level.
Special learning tools
Gaming - Not video games, but piano games! These are effective in testing and reinforcing musical literacy in a way that doesn't feel like a test.
Piano Maestro - The interactive nature of this app makes it enjoyable especially for young children. Although not a replacement for parental guidance with practice, the app does provide excellent supportive aides for very early/young learners of the piano.