When we boarded the Magic, we first went to make Palo reservations, and then went to Flounder's. We reserved time at Flounder's for semi-formal and formal nights (grandparents were babysitting on Palo night). While we knew we'd probably want to take our 17-month-old additional times, we didn't want to schedule it without knowing what we'd be doing and when.
Three separate times during the cruise we called to see if they had openings right then... twice they did, and once they didn't.
My advice is to make reservations for the times you KNOW you'll want your child there, and the other times you can try to schedule throughout the cruise, possibly even at the last minute. Good luck!