Do it the easy way, and just have fun...
We took a T-shirt to Guest services with our childs name and Stateroom number on the first night. By Wed. we had the front of the shirt all signed with every character on the ship.
Mickey
Pluto
Minnie
Snow White
Goofy
Dopey
Chip
Capt. Hook
Dale
Mr. Smee
Daisy
Wendy
Donald
Alice
Cinderella
Pinocchio
Let me tell you...there is no matching these shirts. We enclosed a Sharpie, thinkng that's what they would use...NOPE! Pure Disney, each signature is in color, a few in black, but just gorgeous.
I'm sure if you made your cards, put them in a ziplock and dropped them off, they would be signed within a few days. And you would have the rest of the cruise to go enjoy/visit those characters you choose to, rather than force yourself to get to all of them.
Good Luck, get a shirt done too while you're at it!