A pony card is where you have a "pony" officially measured for height and the "pony" (which can actually be a really short horse, anything 14.2hh and under is a "pony", over is a "horse") who is 14.2" or under receives a card stating that it is a pony.
It's much better to be the largest pony, than the smallest horse as the jumps are closer together and lower for ponies (I believe, I've never jumped anything.... intentionally) so the largest pony has a distinct advantage over the rest of the class whereas a tiny horse would be at a distinct disadvantage in the horse class with a bunch of giants!
So as a kiddo pony, a pony card would distinctly add to her marketability/value.
I have a 14.2hh Morgan that I am debating on the pony card. From what I understand, the horse (well "pony" in this case) has to be entered in a show for the official measuring to be done. Since I've never jumped so much as a ground pole intentionally, I can only imagine entering in a jumping show, I'd be in the dirt at the first jump. My trainer thinks I can get the local kiddo jumper trainer to have one of her better kids show him (he's a quite green 5yr old) but we'd have to teach him how to jump first, then let the kiddo practice and so forth which all just seems like a pain considering I'm not ever going to sell him but I have 2 small kiddos who'd like to eventually jump, so maybe I should just deal with the hassle and get one.