You know your child the best. If you're testing the waters with his interest in music, a keyboard might be best. However, if she's been begging for months for piano lessons and you cannot keep her off of one when she's in the same room as one, you may want to invest the time and commitment in an acoustic piano.
Notice I don't say invest "money." Pianos, especially upright, are constantly being circulated in any city or town. People are always looking to let go of a good piano--their children have grown up and moved out and no one's playing it now; a nearby church wants to upgrade; a family is moving and doesn't want to take it with them, etc. Folks often want to let it go for free and only ask that you be responsible for picking it up.
In any case, most of the pianos that get sold "for nothing" are in good working condition. They usually need no more than a tuning. And folks who want to let go of their piano rarely try to talk their piano up like a salesman. If anything is wrong with it, they are usually simply unaware. Here are some questions to ask them to help you determine if you are looking at it costing you more than tuning and transporting.
1. When was it first bought?
2. How many dwelling places has it had (homes and or storage)?
3. Were the places it's been climate controlled (i.e., central air and heat)
4. Do all the keys work properly?
5. Does it have a humidifier and or dehumidifier system in it? (you usually don't find that on used uprights but may as well ask.)
6. How often was it tuned and when was the last time it was tuned? (You need to tune your piano every 6 months, just like a dentist check up. The longer it's been since it's been tuned, the better the chance it will be harder to tune to concert pitch (A=440)
7. How often was it played and who played on it? (Pianos, like all acoustic instruments, like to be played--it keeps them "alive". :-)
Then if it passes this criteria--or most of these questions--you'll want to go out and play on it. You may send me its description and I'm happy to give you my opinion..
I suggest finding some on the web, then spending a day going around and playing on them. You can also go to a used piano store and play on those to get an idea of what it's like to try out a piano, before you go to individual homes.
Feel free to email me more questions: Michael@musicwithmichael.com