As a prominent Registered Piano Technician (RPT) in the Greater Philadelphia region I’m often asked if it’s a good idea to start a beginner off with a cheap or free used instrument from Craigslist.
No, with very few caveats, no.
Economically, you save nothing as this video shows what i am often inspecting.
In addition to the money, without fixing the problems of a 50+ year old instrument, you are setting your child up for failure. Aside from not sounding right, the keys won’t allow them to learn the touch required to go from pressing “Buttons” (keys) to creating music.