Hi Tyler,
You could bring your 9 month old on
Toy Story Mania by having them sit on your lap. I have to say that it would be somewhat like trying to play ring toss at a fair while holding a squirming baby and wearing 3D glasses. In other words, you wouldn't be able to really get the most out of the experience.
During Toy Story Mania, you really need both hands. One holds the "wheel" on the "cannon" (the thing that shoots virtual darts, rings, etc). You use that hand to aim at the targets. The other hand is busy pulling on a string that shoots the virtual darts, rings, balloons and more. If you are also holding a baby, either the cannon won't be aimed or the string won't be pulled...something won't be getting your full attention. Maybe you'll luck out and your baby will be asleep on your shoulder?
It's worth it to try riding with a baby anyway. Toy Story Mania is a fabulous ride and it's fun to see, whether or not you get to fully participate in the experience. Besides, I'm sure you've held your baby on your lap while eating before, so you've got some experience juggling already. ;)
Good luck and enjoy your vacation,
Kaylene