As much as I like Lucina and co, they have their own game. They only put Owain, Inigo and Severa in Fates because of the popularity poll and I think that's enough. Besides, with all the DLC we got in Awakening, there is no need to do another game just for them (ok I lied, I want to play the ''escape to the past'').
And please, don't even talk about the children in Fates. I hate them already.