When I was writing that story, I made a bit of research about children abused as soldiers. I think Crows would be almost as bad - in some things perhaps less cruel, but only because of efficiency sake, not because they care for children.
While little Crows are not trained immediately as assassins, they are trained as rogues. Both Zev and Leliana say that it takes years of practice to learn basics. This training would start as soon as they are purchased, together with a lot of exercises to build up strength, stamina and pain endurance. When they are 12 - 13 the second phase of their training starts. They are assigned to a Master who trains them as he wants/needs, also according to their individual skills. This phase lasts approx 5 years. The training ends when they are about 17 - 18 and it is then when they are sorted into teams, based on their skills.I say 'team', because I think 'cell' is something more.
Hierarchy, as I see it is like this
'House' are noble houses. At their head is a noble who 'inherited' one or more cells Crows, and he gives them contracts that are in his interest. But the noble is not a Crow, nor has direct power over the Crows.
Cells are governed by Senior Masters, higly experienced assassins, such as Ignacio. They ofthen govern all Crows of a city/country other than Antiva, i.e. Ignacio is responsible for all Crows in Ferelden.
Cells consist of several team, with the Master at the head of each. Teams include assassins, but also healers, herbalists, mages... Members of the team can work individually or in small units (like Zev, Taliesen and Rinna).
Within the cells and teams there is high competition. Everyone tries to get the better and more difficult missions, in order to make their name and get promoted. Crows can be promoted to the Master level after several years of successful high-ranked missions, usually around 25 years. My Zev, for example, needed one more successful mission before he would be considered for promotion (but if it was assassination of the GW, he would be promoted for sure).
To become the Senior Master is much more difficult and takes more time, because that requires also skills in politics and diplomacy. Normally it is around 40. Each Senior Master works mostly for himself, trying to get as much influence as possible, especially those in Antiva. When the Guild Master dies, the one with the highest influence and power becomes the new one. The position of the Guild Master is very difficult - he coordinates cooperation of different cells, sorts the conflicts between the Houses, sets the strategy of the Guild in the next period (e.g. that some House will not be supported any more), and is responsible for finance.
But teams and cells can cooperate if necessary, and the Master can assign a mission to an assassin from another team - of course, the assassin's Master has to approve it. This is normally done only for exceptionally difficult missions... or when they want to get rid of an assassin who could soon endanger their own position of a Master.
The costs for purchasing and raising a Crow are not very high. The assassins are not paid for the missions they do, they only get some pocket money plus wine, women etc. But they don't get money untill they are promoted to the Masters. And as there are always enough slaves and orphans, the lives of an assassin doesn't mean that much and the Crows don't have a problem to get rid of him, if he causes problems.
Uh... I think I'll stop here. Sorry for the long rant.
Going back to my Harry Potter crossover now.