Wellthatsit
I use Wunderkeys (one-to-one) and am considering taking a closer look at the Kiddykeys programme (small groups). They are both what I would consider 'pre-piano', so the children are learning really basic things - high/low, loud/quiet etc. rather than learning to play tunes. While this might seem like they aren't learning very much, they really grasp the concepts and the Wunderkeys programme also works hard at developing early numeracy and literacy skills - e.g. counting the correct number of notes, reading from left to right, recognising patterns.
After Wunderkeys, I use the Alfred's programme called "Music for Little Mozarts" which is geared at really young children from 4-6 (roughly - for me it works out at reception/year one to year one/year two depending on the child). Again, progress is slow but I focus a lot in these early stages on technique, hand shape, legato playing etc. Because the tunes are easy, the children aren't just memorising them but develop good sightreading skills and good playing habits. There are also general musicianship aspects to the course.
I've noticed that a 7yo who has been through the Mozarts programme will have better general musicianship and sightreading skills, and play more musically, than a 7yo who started at 6 and did the other courses I was using and has 'caught up'. This may be because the more naturally inclined child started younger, but I think it's because starting younger and moving more slowly has enabled them to really focus on the basics.
The Wunderkeys is just the cost of books, with free teacher registration and printable resources that you can laminate. MFLM is quite expensive but it's worth getting the CDs, flashcards, toys etc. because they round out the learning experience (I initially only used the books but thought better of it!).
For Kodaly relating to MFLM... you can always clap, read the notes, read the finger numbers, sing the song if you want (letter names, solfege, words), before trying to play. I have some students who do more of these than others.