i always advise parents who are getting their children into piano lessons not to invest a lot of money into it straight away, the last thing you want is a $5000 piano that's going to sit in your living room and not be played, however, pianos do make nice pieces of furniture.
first off i loosely recommend a keyboard that is in the $100 to $200 range to get them started and to go easy on your bank account. at the very least it should be touch sensitive meaning the harder you play the keys the louder it gets likewise the softer you play the softer the sound. this is an ok place to start when trying to get started but not a good place to stop. you don't want them to develop a bad technique which they will by spending to long on a imitation instrument, once they are showing adequate interest in piano lessons and they're going to keep at it, it's time to invest in an upgrade. if possible skip the keyboard and move straight to a digital piano.
the next step up is a keyboard with weighted keys. this means that the keys feel heavier to play mimicking the action of a real piano. on an acoustic piano when you press a key it moves a hammer and strikes the string/s, because of the mechanics, it gives the key a weighted feel. now days keyboards are fairly close to getting the 'feel' right but in my opinion nothing plays the same as an acoustic piano. when you buy weighted keys it will always be touch sensitive.
when purchasing weighted keys you're more than likely buying a 'digital piano'. these are cool because they usually come in a frame with pedals and the appearance of a real piano for about $1000 give or take depending brand and other features. this is a good place to stop and sit for a while if you don't want to jump up to an acoustic piano. the other advantages to a digital piano is that they're easier to move, they never have to be tuned and they have volume control if you need to be quiet. the disadvantage to a student who may be easily distracted is that they come with all these other sounds and additional features which can eat up practice time by playing around if not under the close watch or ear of a parent.
my last note on buying a piano is don't always think brand new. pianos (depending on the brand) are built to last, it's not uncommon to have a piano that is 30 years old that looks and plays like new after being refurbished by a piano store (not everyone who sells pianos refurbishes them). pianos, even though they may be old may not have seen a lot of playing time and may still be in great shape especially when fixed up by a pro.
when in the market check out rick jones pianos, when you purchase a piano they'll trade in your old piano for what you paid for it towards an upgrade.
for my needs, space and financial limitations as a piano teacher i teach on a 50" Yamaha U2 upright piano. before purchasing an instrument on any level do you homework and talk to a professional (not necessarily the guy trying to sell it to you). piano tuners are a good source because they're the guy's that fix them also you trustworthy piano teacher may have a few ideas :)
