Provocative questions 
Finding the right answers to the following questions is not important. What's important is that by noticing these things and wondering about them, your child will begin to make connections, understand how music works, and use the colors and topography of the keyboard as the guideposts they are intended to be.
 
Sit at the piano with your child and ask:
What do you notice about the look of the keyboard? 
What would happen if there were no black keys, only whites? 
Do you notice anything about the sizes or feel of the notes? 
Why do you think the black keys raised while the white keys are flat?
Do you see any patterns the way the keys are arranged? 

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The Musical Explorer
Random, child-initiated explorations should be encouraged. As long as they are kind to the piano, the beginner's natural impulses reign supreme. When your child asks for more, try these lightly-guided explorations: 
 
Color-coded explorations (bravery level 1)
Try playing only the black keys. You might go in one direction, the other, or mix them all up. What about playing two black notes at the same time? What do you notice about the sound? If your instrument has a sustaining pedal, hold the pedal down with your foot while playing the black keys. Do you hear how the notes blend together?
Try all of the above with the white keys. Notice a difference?
 
Intervals  (bravery level 1.5)
Intervals are any two notes played together. Different combinations have distinctively different personalities. Certain pairs sound tranquil while others sound irritated or mysterious. Their relationships can be likened to a playdate with two friends...
Play two white keys together that are within an 8-note range
Do the keys sound as though they are getting along or having a conflict?
Try different pairs, one at a time, and in each case, notice, i.e., are they sharing the toy or fighting over it? 
As you become familiar with the different sounds, maybe you could make up a story about two friends?

Chords

On the TV show "Who Wants to Be a Millionaire," a fellow lost $500,000 because he didn't know that a chord is made up of at least 3 notes. At least this will never be your problem. 

Using the diagram above, eyeball the keys C, E and G. 
Using your left hand for C and your right hand for E and G, place one finger on each key without pressing them down. 
When all 3 fingers are in place, press the 3 keys down at the same time.
You have just played a chord!
Notice that keys are skipped between each note. 
Using any white keys (for now) try playing chords in different places on the piano, i.e.