There are learning keyboards that you can purchase. Notes and tunes light up on an LED display. They learn the names of the notes, and also how to play simple tunes. Most come with a recording feature and they will have a blast listening to their masterpieces. Just google learning piano or learning keyboard. As a former music teacher with a minor in piano, my advice would be to hold off on the formal lessons until they are about 8 years old or so. They will need to be able to have basic mathematical skills, such as adding, subtracting, basic division, etc in order to comprehend basic theory, ie rhythm, beats, note values, time signature, etc.
In the meantime, get them a good portable, and let them experiment in developing their ear and creativity with a piano. Picking out even the easiest songs ("M. Had a Little Lamb", "Row, Row, Row Your Boat, etc) goes a long way in getting them ready for starting lessons. I took piano for 16 years including coursework in music school, and this was how I began playing myself.
Music is so wonderful. Kudos to you for wanting to build on their musical talents and interests!!