You can teach the piano to children from age 3 onwards using the Dogs and Birds method.
It is not necessary to know the alphabet.
Animals symbolize the notes: Dog for D, Bird for B, and Cat for C etc.
I would, without hesitation, recommend "Dogs and Birds" as a most wonderful introduction for our youngest children in the commencement of their musical journey at the piano. Emily Jeffrey – Royal College of Music, Purcell School, and Royal Holloway University of London
What People Say About the Dogs and Birds Piano Method - See some video testimonials here.