Dayna:
Most cruise lines are very strict about adhering to the age criteria in order for kids to participate in the program. You'll have to ask the youth counselors once you're on board if they'll let her participate -- they may say yes, but be prepared if they don't. Disney does have a nursery for those under three years (you have to sign up for it once aboard and pay an hourly fee) which could be your back up if she's not allowed in the youth program.
Disney is a great family product and they'll be plenty of other activities too, such as character breakfasts, tea with Wendy, etc.
Smooth sailing,
Luisa Frey Gaynor
Family Cruise Editor