I know Olivia is the widely preferred pairing, but I've heard that Sumia makes both children exceptionally tanky with Aether, Rightful King, and Pavgis. It would also free up Henry's seed for Lissa. But is if worth it? Also, what do I do with Olivia?
Both pairings are really good and both have their strategic uses. In general, I view Chrom/Olivia as more for Cynthia's benefit and Chrom/Sumia as more for Lucina's benefit, since the former frees up Cynthia for another father and the latter gives Lucina a fantastic skillset. It really depends more on what you want to do with them, though.
42 Str for both and are the slowest/strongest class they can use to outrun Anna w/o a Swordmaster pair up or AS+2 reclassing for more power to a slower class and they'd have to sacrifice a skill slot to be able to beat TSON's Anna.
Comparison Sumia!Lucina Paladin has -1Skl, +1Def, +4Res, +2Mov
And Sumia!Lucina does better Magically and Tank better as well.
Cynthia then becomes a Lucina clone w/o Rightful King.
Sumia!Lucina is the best user of Rightful King.
Chrom!Inigo isn't really too great as Inigo really gets nothing from Chrom and RK is a waste of a slot for him.
I've heard Virion x Olivia makes a good Inigo, but I haven't tested that myself yet.
He does, actually. Access to almost all the breakers and almost all the faires make him very flexible in a variety of roles. Those caps are also really solid as well. Though I'd argue it's more of a testament to how easy Inigo is to make good than it is to Virion's superiority as a father.
Chrom!Inigo is actually pretty good, but not for Rightful King. It's for Cavalier access, giving him Luna and Aegis, with enough SPD that he's flexible even in the best DLC (something Frederick, Stahl, and Kellam have varying degrees of success with themselves).
If you go with Virion he's still fast, but he's stuck with Sol or Astra.
Chrom's totally a legit father for him, and makes an interesting offensive/physical support Lucina. Overall, I'd say that probably Sumia x Chrom is better, because Inigo can be good many other ways and tying up Sumia's limited issues gives everyone else more flexibility. But Henry!Cynthia > Chrom!Cynthia by margins... but probably Henry!Brady & Chrom!Cynthia > Henry!Cynthia and Whoever!Brady.
It's a tough call. If you really want a godly Lucina though, you may as well make a female Avatar and marry Chrom yourself. RK Morgan is basically Chrom!Inigo without any downsides (not that Inigo actually has that many himself), and you still get a good Inigo.