Begin with the notion that background music is delightful. My own son often hummed without any prodding or 'reason', repeating little tunes from his cradle and playpen. (At 18 he is still humming (now it's his playlist of different genres of classics!)
When your toddler or young child plays before and during the bath time, add in the background of the CD. Perhaps you rotate it with other favorite music.
Routine car trips to and from the market, school, the park, provide perfect times to listen. Not so loud you cannot talk, but in the background. If you manage regular cleanup, pickup times in your house, the CD in the background can turn into more dancing and physical play fun. Please do not plug ear buds into the child's ears, this tends to isolate the child. Instead, remember that this adult driven effort to listen together is a gift: you include him or her in a wonderful activity which requires no 'effort' of the child. This easy absorbing as with the language you first spoke around, to and eventually with your child, is an environmental feature of your family life. Later, when a middle school age piano student is memorizing something, this deliberate individualized focus is of course useful and a feature of the quiet listening with ear buds!
Becoming familiar with the tunes and rhythms in Book I is as important for the parents as for the child! If the music is often 'just on' during times when the child arrives back to the home, it becomes part of the fabric of the household. No effort required.
WHO instigates the listening? Always the parent! We cannot expect the child to setup the environment at such an early age. The idea is to listen as often as possible, and with little fanfare, expecting nothing and getting everything. As children grow up, we often cringe or gloat when a pet word or epithet is repeated by a child in the perfect context. We can't remember 'teaching' it to them. Everyday environment is where they absorb.
WHAT if my spouse or other family members get 'tired' of listening to the Twinkles and Lightly Row etc? Take a moment and explain that this background listening is creating a huge desire to play a difficult instrument and is giving 'body awareness' of these tunes. Then put on other CDs! #2 or #3 to vary the songs your child will later be itching to sound out on her own.
Exactly what is body awareness? When you instinctively tap your toe or rock back and forth to a great piece of music, you're feeling it. The physics of music in our body will be topic of another post - a great discussion of the interplay of art and science which results in variations of timbre on a keyboard and therefore the emotional interpretation of music. Made or heard: Croon. Lull. March. Fly. Rock out.