Well, I will say that, when doing Lunatic mode, Frederick is absolutely mandatory for the first five chapters--or whenever the outerrealm gates open. Giving him the Avatar's bronze sword and Virion's elixir is recommended, as well as pairing him with Chrom until then. I don't know if you have the EXP DLC pack (called EXPonential Growth) but, if you do have it, grind your characters until they are at a suitable level, which will most likely be relatively high until you can get and afford Master Seals.
Pairings are mostly up to personal preference. Because of Second Seal abuse, all your characters can end up capping their stats, anyway--both parents and children. You can pair who you want and it most likely wouldn't matter much in the end.
I can give a few suggestions, though. I would pair Chrom with Olivia, as Lucina benefits from the Speed and gets the Pegasus Knight class line for Galeforce and Inigo gets the benefits of being Chrom's child as Chrom is a good father. If you plan on using Sumia, you can pair her with Chrom instead of Olivia for a good Cynthia but in exchange for a slightly weaker Lucina. You can also pair her with Frederick for the good defense he provides her and he benefits from the speed she gives.
I hope that helped. Good luck with Lunatic, friend.
