By what I heard, a good amount of the children aren't that good, especially if doing Lunatic runs without grinding. The only ones I really heard good things about were Shiro, Siegbert, Percy, Velouria and Ophelia. Otherwise, the others tend to be either decent or weaker(for Kana, I've heard many mixed reviews).
Than again, I've heard a number of reasons why...
-One is that children only get 3 classes when they are first gotten compared to Awakening who got all their parents class options. Their default, plus 1 from their father and 1 from their mother. As such, some children may get good stats, but poor classes to utilize them. I heard Mitama is a victim of this, where she either gets classes she wants, but not the stats she wants or the stats she wants, but not the classes she wants.
-Another is that some of the fathers have poor stat choices and their stats tend to clash alot with most of their potential wives. As such, their children tend to be similar stat-wise to their parents and, as mentioned, may have poor class choices compared to others that get good stats in the stats they want and classes that they want.
-Last, some of the Gen 1 Fathers have their special weapons, like Ryoma, Xander and even Corrin. Because of their special weapons, they will almost always be better than their children. Although I did hear that Foleo can compete with Leo depending on who his mother is.
