Hello, Stacia!
You'll be happy to know first of all that infants are welcome on any attraction suitable for
guests of any height. This means that even without utilizing the
rider switch service, you and your baby will be able to experience dozens of
Walt Disney World attractions.
The baby will never be required to take a seat of her own. It is fully expected that someone so small would be riding on your lap. As far as utilizing the carrier, it's fine for attractions without a lap bar, such as
"it's a small world" or
Dumbo the Flying Elephant. Attractions with a lap bar, however, may require you to put the baby forward facing on your lap.
I also wanted to mention the fabulous
Baby Care Centers. With comfortable changing tables, high chairs for feeding, a quiet room for nursing or pumping, and all sorts of baby related necessities available for purchase, it's a perfect infant oasis in each theme park. I know lots of moms who just pop in to cool off and have a bit of quiet time with their infant before heading into a busy ride queue or to rock a baby to sleep before grabbing dinner.
Have an absolutely wonderful time sharing the magic with your teeny, tiny princess!
Anna