As I understand it, you would not make a reservation, that flexibility
is one of it's main selling points ; if you have the card just walk
up, there may be a special line for card holders so in some cases
still have a little wait.
Recent change: you DO have to make reservations for the dome even if you have Firenze card, In fact, this disclaimer appeared on the front page of the Firenze Card website when I pulled it up just now:
"Due to the large amount of requests, timed reservations for visits to the Dome are sold out until April 18, 2017"
And under the FAQs:
"You don't need to make any reservations with Firenzecard, except for the Dome (Cupola). For the Dome (Cupola)the booking is required - you have to make it at the ticket office in piazza San Giovanni 7/r after the collection of the free ticket. Firenzecard lets you visit the museums when you want to and you enter directly into the museums through the reserved entrances. Just go to the priority access following the “Firenzecard” signs and show your card to the staff of the museums."
Under the FAQs for children of Firenze Card pass-holders:
"For people under 18 years of age who are not European Union citizens and/or not members of the same immediate family of the Firenzecard holder, they can: 1) buy a Firenzecard 2) enter the museums through the priority access with their parents who are Firenzecard holders. After that: - collect the free ticket to visit the civic museums (Palazzo Vecchio, Cappella Brancacci, Museo Bardini, etc.) directly at the Firenzecard entrances; - collect the free ticket and buy a reservation to visit the state museums – that are paid directly at the Firenzecard reserved access entrances (price € 4.00 for the Uffizi Gallery and € 4.00 for the Accademia Gallery); - collect the free ticket/purchase a reduced/full ticket to visit the other museums, according to the various museum admission rules."
Do read all of the text above for notes about the € 4.00 child's fee for priority entrance to the Accademia and Uffizi with parental pass-holders, although it has been reported that collecting of those fees is sporadic: sometimes happens and sometimes doesn't.
Does this help?