Who you marry doesn't matter. Almost all the children characters are completely pointless for the main game. My advice is, grab Second Seals every chance you get and use them prudently on your most useful units.
Your twisted logic is that of an immature, egotistic brat! - Naoto Shirogane
Well, on a first playthrough, you really shouldn't worry about min/maxing and optimized marriages; just have fun.
However, there are pairings you should avoid like the plague (mostly due to serious class overlap for the kid). I'll give a list:
Fredrick x Sully (total class overlap) Stahl x Sully (only gives archer) Lon'qu x Sully (only gives thief) Ricken x Maribelle (only gives archer) Gregor x Olivia (total class overlap) Vaike x Olivia (only gives thief and fighter) Donnel x Olivia (only gives fighter and lolvillager) Donnell x Cordelia (only gives troubadour)
I'm probably forgetting a few that are pretty bad, but those are some of the main ones to avoid doing.