Amy, I agree with Bernhard that the very first lesson should be at the piano. He's no doubt been told he's going to have 'piano' lessons, if he's NOT playing the piano you could actually have a riot on your hands (depending on personality and parenting) or he may not come back (depending on personality and parenting....

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I would do something pretty similar to what Bernhard is recommeding. Just let him thump away for while. I've been surprised at how this works with different kids. Some will be very happy to do this until their parent drags them home, explaining, 'but it's the next girl's turn." But I've had one little girl who did what she was told, and then within about five seconds she very neatly and quietly folded her hands in her lap and looked up at me. I told her she did very well and to have another go. I do this a few times, with the same result. So we moved on to another activity.
I've found you can't take anything for granted these days with kids, such as just assuming they know what black and white is. Break concepts up into very small guided stages. For example, you want him to notice that the piano has black and white keys. You could ask:
1. "Can you see how many colours there are on the piano?"
2. "What are the colours?" (If he doesn't know tell him.)
3. Get him to show you where the black ones are and where the white ones are by saying, "play some black ones, play some white ones."
4. You can then play a single note and he has to tell you if it's black or white. Keep things very energetic, very quick and obviously a 'game' with lots of exaggerated praise when he gets it right. I find that setting very easy achievable goals is very important with young children at the beginning stages. They have to feel that HEY!!! I can do this! And I want to do it again!!
5. Now you want him to notice that the black and white keys are in a pattern. Ask him to count (as loud as he can!! - most little boys like yelling) 1 2 3 as you play the black notes. Repeat this in other groups (3-5 times),
6. Ask him "Can you find a group of 123 notes?" Lots of excited encouragment when he does, melodramatic uh-oh's when he gets it wrong, ask him to look again and give him time to correct it. If he still gets it wrong, show him and send him on the task again.
7. Tell him he's going to have a race. Tell him to stand up and get ready, because as fast as he can he has to find all the black notes, from down here (show low notes) to all the way up here (show high notes)and if he gets it all right he gets a prize (if you wish! I like giving kids things, but that's just me

) Ready Set ..... Go!!! and make sure you get completely out of the way (and the piano bench too) 'cause some little kids nearly do run up and down the piano (and I've been bumped into before, so I know now to take a BIG step back).
8. Play the same game, going the opposite way.
9. Calm the lesson down now, (after you give him your prize) and tell him about low notes and high notes. Show him on the piano, and then get him to thump low, and then high. Now he's ready for another game.
10. Ask him to stand beside the piano, and if you play low notes, get him to crouch down really low to the floor (it's best to demonstrate) and when you play high notes, he has to stretch as high as he can and wiggle his fingers in the air (again, demonstrate). Play this a few times, and then introduce middle sounds. When he hears and sees you play in the middle, he has to stand with his legs apart and his arms folded in front of him. Now he has three positions and sounds to play the game.
11. If you still have time, tell him to sit at the piano (I've forgot to mention, I do all these games so far sitting beside the child. There's a few reasons why I do this, but one important reason at this stage, 'cause 4 still is pretty much toddler, is for bonding and security issues. I want the child to learn to trust me and know that I'm on their side and will help them lots. So hopefully they'll listen and cooperate with me. It's worked so far.) STORY TIME!!!! And make up a short story about animals, and he has to provide the sound effects. A little bird goes, and let him do what he wants. The big fat elephant goes, etc.
12. The last thing I do is make up a song (yes people a SONG to be sung with words). Ask him if he has a pet, and what the pet's name is. that's what the song will be about. Make sure it only has at the most four little phrases. Here's an example of a song we wrote recently, "J-O-K, J-O-K, my dog's name is Jok." Sing it while you play something, get him to sing along with you a few times. Then write the words in a book. At 4 I'd be a little blown away if he could read fluently, but hopefully his parents can! As you both sing the song again, get him to thump away as he sings. They usually thump out the rhyhtm. Don't worry about notes, and if he needs a suggestion, get him to use the three black keys and hit them with his fist. Play the song again in different registers of the piano.
13. When it's time for him to go, show his parent what he has to practise this week. Yes! Practise from the very go! Demonstrate the song and the process and tell the parent what they have to do. They should be aware that at this age, the boy needs to practise with a parent.
I expand on these ideas through the next few lessons. Slowly introducing name notes, practising writing A B etc on paper (cause they haven't done a lot of this at 4 either). Note reading, staffs, clefs etc I don't introduce until much later.
Aural skills are more important to develop at this stage.
Just 'listening' can cause a lot of problems. Kids don't want to just listen, and if you are going to have him just listen, make sure it's not for very long. Even a minute is a long time for a little 4 year old boy excited about being a piano player to have to try and listen to something! DOING, being ACTIVE (yes we know listening is active, but not so much at 4 - most 4 year old think of having to listen to something for any amount of time as sitting still and being quiet. Listening AND visuals work better.)
Anyway Amy, hope it goes well for you and your 4 year old. I think this age group takes the absolute most work for a teacher, but it can also be the most fun and rewarding. (IF you like kids I guess....0