From the Disney Cruise Line Blog:
The following is a summary of the offering DCL is providing travel agents.
Pre-Book Character Greeting Opportunities Aboard Disney Cruise Line
Beginning on Tuesday, June 7, 2016, your Clients will have the ability to pre-book two Meet & Greet opportunities with Disney Characters through the My Disney Cruise section of DisneyCruise.com. Guests sailings on cruises of 7-nights or longer that feature at least two sea days will also have the opportunity to pre-book a special Character Breakfast online.
Similar to Port Adventures, if your Clients do not pre-book these experiences prior to sailing, they may check availability once onboard.