@Ren: Never read Cloud Atlas but interesting to know the name/number thing has been used before and works.
@Merrybells: It's a sort of dystopian Earth - overpopulation means every family is restricted to two children, these 'army kids' are the surplus births that have been taken from their families when not given up voluntarily and trained as military to keep the nation safe. They're treated as second class citizens because they legally shouldn't have even been born.
@Stardust: I haven't quite decided on the ages yet. They're going to be old enough to look after themselves but not too old - probably somewhere between the ages of six and ten, or a mixture of ages in that range.
You've definitely all given me a lot to think about.
I don't think you should give them degrading names; something completely neutral would be the best. You need something no one would have put any thought into at all. They are not considered persons, so they don't need something to show individuality. I think the name of whoever found them is a good idea, or where they came from (this was quite normal in Ancient Rome).
Agree w ottilie. It's an interesting concept. I also would NOT use numbers, especially long numbers like 479, since the average reader will not remember them and won't even be able to keep up with the cadre of six main characters let alone accessory ones.
In real orphanages like this, the naming was entirely impersonal. Take religiously-run institutitons; children were named according to the calendar of saints based on the day they were presented. Modern overcrowded Chinese orphanages simply name the children Baby Girl 1, Baby found on Tuesday, or equivalent. Obviously you won't have a religion factoring into this world but could you have some sort of state calendar with illustrious heroes of the people or something according to which the children are named? Again, they need names both for your readers' sake and for verisimiltude in this world. Also, you could try naming them for the streets/buildings next to which they are found/dumped. That probably wouldn't be realistic though as most of the children would be dumped at the gates of the orphanage itself.
It would be fun to have just 1-2 redundancies-- you know, two children who entered on the 6th day of the 7th month so had the same name, just with an identifier attached (physical or numeric).
And they could all have the same surname based on something militaristic, or related to the name of the state (to show they belong to the state).
I still think having real names keeps the story more personal and in-depth, but that's just my opinion. Also, what if their names came from their numbers? Like, if your name was Seven Eight, phonetically, you'd be sevenate (yes, I know merrybells used this), but then, you could become Nate (sevenate) and if you're just Seven, you'd become Evan.