I figured since I'm going to main Robin I'll play the game they came from. Also I want to make Lucina my waifu
There's an abundance of girls to choose from, and their children gain different skills and stat growths depending on their fathers.
The system is more than aesthetic, though that's what most people (including me) use it for.
Personally, I married Olivia and my children (Inigo and Morgan) ended up being speed demons that mainly use magic.
I actually hope you change your mind on who to marry, because I personally find marrying one of the children weird, but that may just be me. There's a lot of girls tho, so take your time and pick your favorite!
If you're set on Lucina, you get her just after chapter 13 I believe, as a lord at level 10. She will be underpowered at first but there's always grinding!
EDIT: If you want all the units make sure you marry every lady who can marry, as they are attached to the women Lucina and Morgan aside. Also do not forget spotpass characters that can be gotten from bonus maps at the endgame! (To get them, go to your online/spotpass menu and look for bonus maps!)