"Prince Morgan" and "Prince Inigo" if Chrom is their father. Less so "Prince Brady", "Princess Cynthia", and "Princess Kjelle", as those names see much less frequent use.
Prince Marth, due to character limitations, was localized as "Pr. Marth". Generally he'll be referred to as anything else that contains those two letters; the most common interpretations are Professor Marth and President Marth, but imagination has been people's only limit - there have also been Public Relations Marth, Prudish Marth, Prosecutor Marth, People's Republic of Marth, et cetera.
"Disney Princess Henry" has managed to catch on with Tumblr, for nebulous reasons.
Those critical of other fellow fans of reducing male!Morgan's whole personality and value to his affection for his mother have given this shallow interpretation of Morgan the nickname "Mama Avatar's/Robin's Pet Poodle". In contrast, the less maligned girl!Morgan is nicknamed "Daddy Avatar/Robin's Right-Hand Kitten".
Fluttershy for Olivia, due to being a Shrinking Violet who's color scheme involves yellow in the body (well, clothes) and pink hair. Make her an animal handler, or Fluttershy a dancer, and she'd be a very close humanization.