The Transition From Numbers To Notes
The transition from numbers to notes can take days or years. It depends on the age of the child and their temperament. In either case, don't rush it. It's likely your child will be almost immediately bored if they start piano with reading music as the only activity.
You can soften the blow of reading music if you use numbers first. This will introduce the child to the piano keyboard in a physical sense. Children who start with numbers get a good start that is both educational and fun. The immediacy of the approach appeals to the child.
Perseverance Requires Interest
Learning the piano takes years. One quality you'll need more than anything else is interest and the desire to keep learning. If you're not interested, you will not survive lessons long enough to learn to play much of anything. Thus, anything that increases interest and promotes the longevity of lessons is a good thing.
As for the transition from numbers to reading music, here are a few points. To a child, it is easier to understand numbers at the piano than deciphering even the simplest of sheet music. Yet, if you never start the transition, it will never come.
Use numbers during the transition as relief from the tedium of reading music. This dual approach begins to build their music-reading skills while preserving their enthusiasm for simply playing music.
Don't Restrict Kids To Reading Music
It is foolish to restrict children to only the music which they are capable of reading. It makes as much sense as restricting children to the foods they are capable of cooking. You need to keep the diet wide, with an emphasis on fun interspersed with work.
Children seek music, and don't care what format it is presented in (numbers/notes) as long as the presentation is not tedious. Presenting notes is inherently tedious. Numbers are not. Be like a waiter who knows exactly when to serve each.
Ratio Of Fun To Work
Reading notes forces the focus onto the page, which is not as interesting for a child. What attracts them is the magical physical logic of the keyboard by itself. Keep the proportion 8 to 2, that is, 8 parts fun and numbers to 2 parts reading music. Raise the proportion if the child is not frustrated too much by reading music.
The ratio cannot be 0 to 10, that is, all reading music and no fun. Or all fun and no reading music. Mix it up. The main rule of introducing reading music is this. If it seems too hard, go slower or switch to something else. You can't continually deliver confusion and frustration to the child and still expect enthusiasm.
Where Do I Start?
Begin with finding Middle C, on the page and on the piano. You need to find the key Middle C, and also what the note Middle C looks like on the page. Until a smaller child gets this in their brain, there's no place to go.
Obviously, older children learn this more quickly, but it's still the first base that must be conquered. Reading music is the goal of piano lessons in the long term. But if you have to trade it for enthusiasm and interest, it is a hollow victory.