As a character:
Say'ri - I'm mostly upset that they didn't flesh her out more; she's rather dull.
Gerome - Probably my least favorite. Just get over it and make use of the opportunity you have.
Brady - I guess I just don't get him. I don't hate him, but while I understand him trying to be different from Maribelle, I don't like their direction.
As a unit (assuming base class):
Ricken - Aside from starting C in magic, he just feels inferior in every way to Miriel. So useless.
Brady - Unless you make Donnel his father for Aptitude (which I guess is the "correct" pairing given his manner of speech), he seems to be the worst child in the game - by a lot. Doesn't excel at anything.
Stahl & Sully - They just seem lackluster compared to some of the cavaliers of the past. Or maybe it's just that specialized units feel more powerful in this game than in the others.