Keeping someone captive and/or forcing them to work for you unpaid or under harsh conditions would count as slavery. More common than you might think. Modern day slavery in the UK A community whose members don't appear in the usual things like electoral rolls & school registers would be an easy place to hide someone without arousing suspicion.
I agree with CogitoErgoSometimes preventing a child, for example being registered with a school, or their entitlement to free school meals keeps them invisible and therefore more vunerable to exploitation and abuse. Nothing wrong with an alternative lifestyle as long as fundemental rights to services are taken up on behalf of children. Sadly there are examples where this is not the case. I have worked in a school where traveller children are sent periodically, without a packed lunch and no arrangement for the free meal they would have been entitled to. It is not always lack of awareness on behalf of their community, lots of resources are put their way. A child never went hungry by the way, at least not on my watch. Those children arrived scared and uncertain and hungry, certainly in no condition to learn. You can see where the danger lies if you fall through the net.
I would class the practise of taking girls out of school at 11 or 11 for them to clean the house and raise the children akin to slavery. Just because they've been conditioned to beleive it's what they want doesnt make it any less so.
As Driftwood said, fundemental rights to services need to be taken up on their behalf, only when they are of age should it be legal for them to decide if to live their life as someone's skivvy.
As for that link, wow, and 'normal' men still use brothels and strip clubs... baffled.
I drove past that site the other day and I watched Big Fat Gypsy wedding 2 + 2 definitely equals 4 for the locals who watched kids taken out of school at 12 to skivvy for the rest of their lives. Its a fairly brutal way to break the practice but ANY social group that deliberately keeps its women illiterate and in servitude deserves breaking, wherever in the world it happens