Opinion on every kid:
Lucina: Married her the first time because of Aether Morgan, but probably won't do it again even if it is a great skill.
Kjelle: She's just... meh. he is more liked than the others, but... just meh.
Cynthia: She's fine. She's likable like most of the children, that's about it.
Severa: One of my most used units, so I've grown to like her.
Nah: I never found her that great. I don't really know why people say she's the most broken unit in the game.
Noire: She's pretty good if married to Donnel! (Yes Donnel, not Gaius.) She can have good strength or magic, so she's a somewhat lesser version of Robin.
Owain: Great in supports, but I messed him up in my first play through, so I barely use him. I haven't gotten him on my second time through yet.
Laurent: Not that great, someone's better than him in almost every way. Can you guess?
BRADY!: Basically Laurent + Galeforce. He's absolutely amazing along with Emmeryn.
Yarne: Seems like he could be helpful, but I only use him for supports and not a lead, so I don't really know. X(
INIGO!: Tied with Morgan as my favorite and most used child. I married Olivia on my second file so Inigo and Morgan could be siblings! ^.^
Gerome: He's almost always Morgan's partner even though they aren't married. Gerome has massive movement and strength, Morgan has massive magic and great skills.
And here she is! Morgan!!!: The ACTUAL most broken unit in the game. (S)he can learn absolutely any skill. Any and all skills. How is that not amazing!