There is a big difference in sound’ satisfaction between playing a piano and a keyboard. You can pound on a line of plastic keys that bounce back up at you, or you can pound on wooden keys that have resistance (is what allows you to play dynamics) and substance. 

The sooner a student has the opportunity to practice on an acoustic instrument with that kind of nuanced musical responsiveness, the better. 

Digital pianos, which are smaller but still have 88 keys, less expensive, and do a decent job of mimicking the sound and feel of an acoustic instrument, could be another option. Please, no keyboards.