I very much recommend "Dogs and Birds"  (www.dogsandbirds.co.uk) as a method for introducing the piano to very young children.

Appealing wooden animals which children want to play with, mark the different notes of the piano, and many interesting games can be played to orientate the child around the piano keyboard.  Then the same animals appear within printed music notes, so it is easy for the child to link notes on the printed page with notes on the piano.

I have never looked back since I discovered this method, as almost all my students are in the 4 - 7 years old  age range.