Why Grand Pianos Are Better Than Uprights
Why grand pianos are better than uprights can be answered in one word: gravity. Grands are inherently better instruments than uprights for several other reasons. The primary reason is not sound quality. The primary reason is gravity. For those who don't know, pianos make sound by means of little felt covered "hammers," which strike the strings.
In a grand piano, the hammers are below the strings. They move upward and then strike the string. When the string has been struck, the hammer falls back into place by means of gravity. On a grand, this "fall back" happens very quickly. This makes the instrument ready for the next note very quickly.
Drawbacks Of The Upright
In an upright piano, the hammers are standing up straight, in the same plane as the strings. When the string has been struck, the hammer must be pulled pack from the string by a variety of means (springs, levers) that vary from make to model.
All of these methods of pulling the hammer back in an upright must account for the inability to let gravity do the work. Gravity is more efficient, and thus a grand's hammers return more quickly and are readier sooner for the next strike.
Kids See No Difference
All this may make absolutely no difference to a six year old trying to play Scooby Doo on the white keys. But to a young professional or student, knowing which pianos are good and which are better is important. For example, Chopin made his students promise to practice only on the finest pianos, and warned them that if they didn't, they would be teaching their fingers bad habits.
In Chopin's lessons, as in many other studios, the master played and demonstrated on a small cottage upright while the student sat at his Erard concert grand and played. While an upright is perfectly fine for a child, real pianists prefer the grand.
Given the higher price of grand pianos, an inexpensive electronic keyboard may in fact be a better choice to get your child started, rather than an upright piano in decrepit or even less-than-perfect condition. Keys that stick and/or do not play will spell the end of interest in the piano just as a broken toy may get little attention.