I think this is an important topic and so a category structure that makes sense up and down would be important. As a member of the category project, I'll certainly call attention to it and to what I would propose at this point:
Status quo: Starting at Category, the downward trail is Category-->World History--Slavery, There is also a trail Category-->Society
Proposal: Without taking anything away, I'd propose creating a new category under Society named Servitude. I would add Servitude as a higher category for Slavery, and also create a new subcategory under Servitude called Indenture. Now you would have a trail Category->Society-->Servitude-->Indenture.
I haven't checked to see if there is a category for Apprentices, but that was a separate form of servitude, also with a time limit, that could fit under servitude -- and also under work or professions.
A category like British Indentured Servants could go under Indenture. I'm not comfortable having one category for both Indentured Servants and Slaves; I think it would be better to have a separate one under Slavery for British Slaves and then put a cross-link under British Slaves and Indentured Servants to refer one back and forth from one to the other.
A category for Indentured Children would go under Indenture. I think it would be important to distinguish between those who were indentured but never moved, and those whose migration was part of their indenture, but I don't think that would require a separate subcategory, just difference of names under Indentured Children. So if you found a child who was indentured but stayed in England, it would simply be Indentured Children, England, and one who was transported would have the category I suggested earlier, Indentured Children, England to Virginia.
I"m personally conservative about categories, preferring to create only the ones one needs for profiles one has access to. So for the profile that would fit under Indentured Children, England to Virginia, I'd recommend creating that profile, once no objections to this system surface, and then create the necessary higher categories over it so that the finished trail would go Indentured Children, England to Virginia --> Indentured Children --> Indenture --> Servitude --> Society --> Categories.
If this is part of a Project, then of course the categories covered by the Project would be subcategorized under the Project name as well.
I would resist having British as a higher level distinction because the focus is on the type of servitude, and Britain is simply one of many places where it can happen; we need a category structure that can accomodate them all. So if the profile warrants it, Enslaved Children, Albania to Hongkong would have a place. But I would only recommend creating the categories that profiles need; the specificity is good when you have the profile to go with it, but the theoretical possibilities are almost endless.