Hi Penelope!
Toy Story Midway Mania! (TSMM) is an awesome attraction and its always a must do for us! You know your little one better than anyone so I'll give you some food for thought so that you can make the best decision for your family.
The line for TSMM typically gets really long, really fast. If you decide to give it a try, go there first thing in the morning. Jump in line while your little one is fresh and before the line gets too long or grab a FASTPASS to come back later - that way your wait won't be as long. Bring something to keep your 16-month old entertained in line. Its a great queue and there are lots of things to look at and Mr. Potato Head will help entertain you.
Now for the ride itself. As you can see from the pictures shown on the link above, there are two spring-action shooters per car and you shoot at your targets by pulling a rope. You can let 16-month old help you aim and shoot at the targets. But if you are super competitive, keep in mind this will not allow you a good opportunity to get a good score.
If your 16-month old is really active, you may opt not to get a pair of 3-D glasses for him/her just on the off chance he/she decides he doesn't want any part of them and sends them out onto the Midway floor. There is lots of movement on the screens so that would keep him/her entertained even if its a little blurry without the glasses.
The ride also spins you around somewhat quickly between the scenes so hold onto your little one when your ride vehicle is in motion.
TSMM really is a great ride and I think you'll love it if you can get on without a lot of waiting. However, if you can't and your toddler is super active (and some are), you may want to save it for another trip down the line.
Wishing you minnie magical memories :-) Allison V.