This is the job of readjusting all the moving parts to bring them back into engineered specifications. Like voicing, over time, moving parts suffer wear and tear. Eventually they wear so far that the lever system no longer functions at top efficiency. Some keys literally (physically) play differently than others. You will find that some keys seem "sluggish," or like you have to strike them harder than others to get the same amount of volume. The piano loses its dynamic range, particularly in the very quiet end of the range ( ppp ). It becomes impossible to play very quiet sections because some of the hammers won't strike their strings. So you lose a great portion of musical expression; you end up playing certain sections of music louder than you would prefer. Consequently, you don't hear the full beauty of either your playing ability nor the piano's. Certain passages of music played on notes that are our of regulation are difficult to play. Often the mistake is made of concluding that the player, adult or child, just doesn't have sufficient talent to play that passage. But often the truth is, the player does have the talent to play it, but the piano doesn't have the "talent" to do its part. It's the piano that is not keeping its end of the deal. The player is playing fine, but the piano is only giving a 70 to 90 % response depending on what note is being played! No wonder no one every plays really good on that piano! Regulating the piano could make all the difference in the world. BEFORE you conclude, for example, that your child has already maxed his potential . . . that that's all the better he can get . . . GET THE REGULATION OF THE PIANO CHECKED OUT! If it's a regulation problem in the piano, you'll be exceedingly amazed with joy at what you hear coming from your child's playing after the piano is properly regulated.
Here's a simple test you can do. Sight down the keys. Are they all level? Or are some higher or lower than others? Are some crooked, leaning to the side? Then your piano NEEDS Regulation.