Etienne,
It sounds like you're a dedicated teacher, and care about your students. However, why not look at the big picture?
What you should say to parents is 1) encourage them to be involved in their kids' lessons, 2) take their kids to concerts and expose them to music, 3) reinforce the idea that music is a life skill, and a good music student is a good student in other areas, 4) keep their pianos regularly tuned and in repair - all of which I'm sure you're doing.
Telling them about the advantages of a grand vs a vertical is fine, but what you should not do is make them feel guilty by pestering them about buying an expensive instrument you don't really have any idea whether they can afford.