Hi Natasha,
I think you'll be pleasantly surprised to find that most attractions at Walt Disney World are designed for the entire family to enjoy together. Having traveled with an infant before, I can tell you that my baby happily slept in my arms while we enjoyed several of the attractions together. At this age, several attractions that might frighten a toddler don't phase an infant.
There are some attractions that, for safety reasons, have height requirements. For a list of those attractions, click
HERE. This is where child swap will come in handy. At the beginning of an attraction, ask for a child swap pass. You and your older daughter will stand in the regular line and enjoy the attraction while your husband takes care of the baby. When you return, your husband and daughter can enjoy the attraction again without having to stand in a long line. Your 5 year old will love this service, because it will allow her to ride an attraction two times.
Hope this helps. Have a magical vacation.