Hi Kabrina!
Thank you for visiting the Moms Panel!
The good news is that
Phineas & Ferb can be found at Disney's Hollywood Studios, near to Mama Melroses's Ristorante Italiano, and this is the only place they can be found at Walt Disney World. As you enter the park, pick up a copy of the Times Guide to check their appearance schedule.
The Times Guide is also a very good way of checking the daily character appearances for each theme park. These guides can be found at the front desk of all Disney resort hotels, and at the entrance to the theme parks.
If there are any other particular characters you would like to see, it is always worth enquiring at Guest Relations when you enter the theme parks.
Another great way to meet a variety of characters is by enjoying
character dining - there are many of these in the theme parks, and at the Disney resort hotels.
I would be happy to answer more questions if you have them, while planning your first magical vacation at Walt Disney World!