This is more of a Miami question than a Carnival question so you'd probably be better off asking in the Florida forum than the Carnival forum.
Celebration uses Terminal F, for "Fun"
Royal is Terminal A
These are marked on the Google Map for Dodge Island so you can judge if you can walk it. There's always construction going on on Dodge Island and there is a lot of vehicular traffic, especially on turnaround days - so you'll probably have to cross the street a few times and stay alert etc. But I would consider it walkable but it is a fairly long walk as these berths are very big. The vehicles around the pedestrian areas aren't traveling at high speed generally.