With exception of the first Paralogue, all other Paralogues are unlocked by marrying off male characters who have a child character linked to them. As soon as they reach S-rank with a compatible female character, and after Chapter 14 of Birthright , children will be available to recruit.
In all Paralogues, the enemies will scale in level to meet the level of your army currently. However, in Paralogue 1, Mozu remains as a Lv. 1 Villager, and child characters will cap off at Lv. 20 unpromoted. Go too soon and you will have more babysitting to do to get a new character up to snuff, and go to late and while the child character will need very little attention, they could be easily killed by the enemy.
If you wait until very late in the campaign to recruit the child units, they will come holding an Offspring Seal that when used immediately catches them up to your current level.
Paralogues are a great opportunity to collect more experience for your veteran fighters as well. There’s no reason to not complete the available Paralogues.