When am I supposed to start recruiting child units?

[Samz707]

I've unlocked the paralogue for Noire but going in there is a horde of dudes with silver weapons and high stat's that's pretty much suicide for anyone that isn't Thajra/Robin with Nosferatu. 

At what exact chapter would you say doing these is advised?

[Sooks]

The stats of the paralogue’s enemies increase depending on how late into the game the fixed parent joins. So Robin’s kid’s paralogue has the lowest enemy stats, while Cherche’s has the highest. Sumia’s kid and up is where enemies are unanimously promoted, at least on hard mode, I don’t know if difficulty affects this, so if the same isn’t true for your army, you might wanna hold off on all paralogues not tied to Robin, Lissa, Sully, or Miriel (and any females I‘m forgetting) until then. But anyway, in my game, at chapter 18 I was able to nab every child unit, so around that time you should be able to handle it. I don’t know anything about lunatic or lunatic+ because if I ever decided to touch this game again, for some reason, no way would I sit through something that agonizing.

[Aethelstan]

It varies a lot. Some like Owain's are quite easy and can probably be handled as soon as you unlock them. Others like Inigo's are very difficult and should be handled when you are around end game levels. Noire's is definitely on the harder side.

Here's my own ranking

Easy - Owain, Brady, Kjelle (if your staff user has enough MAG to rescue her quickly), Morgan

Moderate - Gerome (easier if you have good fliers, dark fliers especially do well here), Laurent, Yarne (if you side with one faction, decently hard if you fight both), Cynthia

Hard - Nah, Inigo, Noire, Severa (easier if you have enough cash to burn through multiple rescue staves, very hard without that)

[Shadow Mir]

As stated, the difficulty is proportional to when the fixed parent joins. 

Easy: Morgan, Owain, Laurent, Kjelle (these have mostly unpromoted enemies, with a few promoted enemies)

Normal: Cynthia, Brady

Hard: Yarne (if you want the most rewards, you must fight both armies - and what you see isn't what you get), Inigo, Nah, Noire, Severa (this is more due to her recruitment condition, as she must talk to a villager on the top of the map), Gerome