If the Character Breakfast is offered on your cruise, when you go to the Reserve Activities page it will be listed on select sea days under On Board Fun, then under Character Greetings Requiring Tickets.
I didn't check that location when first booking activities for our upcoming European cruise because I didn't want Princess Gathering or Frozen Meet and Greet tickets. When I did look there a month ago, the breakfast was sold out, but more spots had opened up when I looked again last week. So, if you are unable to get tickets on your first try, keep checking as the cruise gets closer.
On our Magic cruise, the listing says "Disney VIP Character Breakfast" so I'm not sure if it will be Disney Junior characters.