That is a lot of people, too many to save, too many to even know about so the question needs to be asked.
Who's dead children don't get raised? Is it as shallow as helping those you see when you know for sure and certain there are more out there dying every day? With knowledge like this can you waste your time and slots raising one child when you could be preventing the deaths of a score more?
Opportunity cost exists.
Easier enough to answer too. Everyone we possibly can under our dominion.
If you're not part of the Imperium then tough shit, death is death, but when you get under our banner you reap the benefits of our rule, and one such benefit is the knowledge of combating the permanence of death and the ability to learn how to preform said rituals.
Provided of course, that you've earned the trust of the state and the right to acquire said skills.
Like for example, say some kid gets eaten by some creature over in King's landing. Nothing we can do about it there because A. We wouldn't know about it, B. Said child is not under our protection by being part of the Imperium, and C. Their protection is the responsibility of the King who's fealty they are demanded AKA the fat fuck currently whoring it up while drinking himself stupid.
Once we conquer Westeros, then yes they are under our protection and as such it is our responsibility to do our utmost for them, but until then they are not.
As for your second point, spending our time raising the dead (or to be more accurate getting someone else to do it for us because we are a King and we need to learn how to fucking delegate) is not the same as failing to prevent deaths.
I'd like to think we're smart enough to do the math or properly schedule our time for such a hypothetical scenario.
Like we wouldn't waste our time raising 5 year old Tommy who was found eaten alive by giant rats while we know for a fact that the Squids are planning an assault on a town on the other half of our kingdom.
We'd get someone else to do that (provided we'd had such a person) and go deal with the more immediate threat.
Let's be real here, we've raised the dead before, our mother is proof of that fact, so saying that we can't or that its too costly is hypocritical.
That it's
inefficient to do it ourselves is closer to the truth, which is why we
shouldn't be doing it ourselves. We should have people who could do that for us. We don't (at least as far as I know we don't) so the choice currently falls on our shoulders, but we shouldn't dismiss it out of hand without at least properly considering the option.
If it costs too much time or resources then yes, the children will just have to remain dead and the families will have to settle for condolences, unless we ever get to the point where it
doesn't in which case they
shouldn't.