I see some interesting pairings here. But if you want to optimize and have the best kids EVER, here are my choices.
Chrom x Olivia – Olivia passes down the Myrmidon class, which gives Lucina great skills like Astra and Vantage. Her mods also make a good physical-orientated Lucina. Of course, you can also use Sumia since her mods give Lucina godly speed and skill. I just think Olivia is better because the Myrmidon class offers better skills as opposed to the Knight class (which Sumia passes down) and her mods are more rounded out.
Now, if we're talking about Inigo, there's no dispute that Chrom benefits him the most. Since Olivia passes down SO MANY classes to Inigo, he usually has the skills he needs. But Chrom passes down Rightful King (a useful skill when paired with trigger skills), and Luna (Cavalier class)! Also, Chrom's mods are really nice for Inigo.
Lissa x Ricken – Since Lissa gimps Owain's strength, a magic-based Owain isn't out of the question. And Ricken offers a whopping 5 to Owain's magic! Not to mention that Ricken passes down the Cavalier class, which gives Owain skills such as Luna, Aegis, and Dual Guard (useful if you want Owain to support). Libra is a second option with okay mods, but he doesn't pass down a lot of classes (other than the godly SORCERER).
Henry x Maribelle – Henry's +4 mag and skl modifiers cannot lie. Also, he passes down Sorcerer to Brady, which is always good. If that isn't an option, you can go with Ricken or Libra instead. Ricken has nice mods, while Libra passes down Sorcerer. Your choice.
Donnel x Sully – Wait, huh? You want me to pair the strong independent cavalier Sully with that sickle-wielding bumpkin from Paralogue 1? B-but his mods aren't very impressive… However, Donnel passes down a skill that everyone and everyone's kid wishes they had: Galeforce. This nifty skill allows you to move one more time after defeating an enemy unit. It becomes quite the skill in Endgame. Sadly, not all children are blessed with Galefore… Unless you make the right pairings. Donnel passes down the Pegasus Knight class to Kjelle (and he can pass down Aptitude, a skill that increases the odds of gaining stats when leveling up). If you don't want that, –shame on you– then Vaike is the next best option.
Henry x Sumia – What people might not know is that Sumia's mods slightly favor the magic stat. Weird, huh? Henry makes the best parent for Cynthia because his mods capitalize on Sumia's (Skill and Speed). Not to mention that Henry gives Cynthia the most classes out of all of Sumia's possible husbands (except male MU). Not a choice? Then Chrom, because he gives his child Aether.
Stahl x Codelia – Stahl offers great mods in all the right stats for Severa (Strength and Skill). He also passes down the Cavalier and Myrmidon classes. Don't like Stahl? Lon'qu is your next option because he gives Severa amazing Speed and Skill. That's all I have to say. Hero Severa is best Severa.
Cherche x Vaike – Gerome cannot get Galeforce. Ever. What a shame… But if you still wanna use this dark knight for your team, then Vaike all the way. Vaike's gigantic +7 Strength mod is not to be ignored. He can make for a pretty cool Berserker this way.
Panne x Gregor – Unlike the other pairings, this one is indisputable. Gregor gives Yarne awesome Strength and Speed to make a badass Berserker. Sadly, Yarne doesn't get Galeforce either, so that's not an option. Berserker is the best class for him. But why not Taguel Yarne, you ask? Taguels… suck. They suck huge. Beaststones only? Range of 1? No forge? No braves? Seriously.
Miriel x Ricken or Miriel x Lon'qu – I couldn't choose here. Ricken gives Laurent that amazing Magic modifier (+6) and the Cavalier class. However, Lon'qu's mods favor Skill and Speed as well as Vantage from the Myrmidon class. This combo works well to make Laurent an unstoppable Sorcerer. Which one do you prefer? Also, last kid that can't get Galeforce.
Tharja x Gaius – Gaius is one of three fathers that passes down the Pegasus Knight class (and Galeforce) to their daughter, the other two being Donnel and male MU. Noire can't get Galeforce otherwise, and Gaius is better than Donnel in terms of fathering Noire. Gaius gives Noire Vantage (always great for Sorcerers) and can pass down the skill Counter (mainly if you want to keep Noire as a Sniper, which I personally don't recommend).
Kellam x Nowi – Kellam capitalizes on the Manakete's amazing defense to make Nah an indestructible wall. Seriously, +7 Defense?! If you aren't into a tank Nah, Gaius is an okay choice since he passes down Myrmidon and Pegasus Knight (only way for Nah to get Galeforce unless you pair Nowi with Donnel or male MU, but Donny's a bad idea).
MU is the best choice for EVERY KID! But there's only so much MU to go around, so choose who you pair up with wisely. Personally, I went with Nowi for Galeforce Nah (and since Nah is my favorite child unit).