Hi Jennifer!
I'm so glad to hear that your kids are fans of Phineas and Ferb, they will absolutely love
Disney Phineas and Ferb: Agent P's World Showcase Adventure! You do not need reservations, and it is not like a typical attraction where you have to stand in line and "wait". When you arrive at EPCOT, head over to one of the recruitment centres located at the Oddyssey Bridge, Norway Pavilion, France Pavilion, Italy Pavilion or by the International Gateway. There you will be given a time to report to your "country" for your mission. Depending on how busy the theme park is and if there are any F.O.N.E's available, you could possible start your mission right away. I have found that we are able to choose what time works best for our family so that it does not interfere with other plans, specifically dining plans we have during the day.
This adventure is one of my daughter's favourite attractions at EPCOT, I'm sure that your children will love it as well!
If you have any other questions about the attraction, or anything else, please send me a follow-up question. I would be happy to make your "Disney Time" even more magical!
Stephanie