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."
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. Feel free to ask me to come with you.
I realize these are a lot of questions, and for a steal--like "giving away" or $100 or something, you might not need perfection. Concert pitch, the sound board* condition, and the keys all working, are probably your most important things to check off your "passed" list.
Sound board-- this is the board of wood that resonates when you play the piano. You want to make sure it is not cracked. When they crack, it usually disrupts the quality of the tone significantly. On grand pianos (horizontal) you can see it easily. On uprights you'll need a flashlight.
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.