We are using:
- Soft Mozart
- Little Musician (beta testing)
- Trembelina
- Tuneables
- Suzie Piano Primer
My son is older (6) but all of these programs could be used with much younger children.
I'm currently evaluating my approach, and if I come up with any other ideas I'll be sure to share

But TeachingMyToddlers is doing an amazing job with her kiddos, search some of her treads and I'm sure you'll get TONS of info.