Who gets to define abuse?
It's the initiation of violence. Are you now going to ask me who gets to define violence?
Can an 8 year old even consent?
A few can, most cannot.
If not, then who can on her behalf?
The only people that can consent on your behalf are people that you have given that authority to, a lawyer or financial adviser for example. However, we can and should treat people how they would want to be treated if they were fully rational. Upon reaching full rationality would she be upset that she hadn't been allowed to get married to that person? If so, she should be allowed to do so. If not, then not. There's nothing special in the case of a child. They are just another person with diminished rationality. It would be no different from stumbling upon a person that's lost a lot of blood lying unconscious in the street. Do they want you to give them a blood transfusion? Probably, unless they are a Jehovah's Witness. In which case, you should stop the bleeding but otherwise let them die if only a blood transfusion can save them. In the former case it would be immoral
not to give them blood transfusion. In the latter case it would be immoral
to give them one. You treat nonrational people how they would want to be treated if they were rational. Is it a custom for women to get married at the age of 8 and the ones that aren't allowed to, live a life of misery and regret? In that case, let her marry. We need to focus less on forcing our personal opinions on others and more on trying to help them achieve what they would want to achieve.
A better way to say it is that the rights of the child are vested in the parents until the child has achieved the age of reason and is able to claim them. But when does that happen?
There's no fixed age. Different people mature at different rates. All age based limits are going to be arbitrary.
Nonetheless, there is no body of law that would say that this 5 year old child who was caged for months in her own crib is old enough to reason. So who says that being caged is a violation of her rights?
Upon reaching full rationality would the child be thankful for being caged in a crib for months at a time? Probably not. Even if so, that would be the rare case and it's better to err on the side of caution and assume that she won't grow up to and realize she's been a masochist since birth.
We can both look at this and know that this is abuse, but what makes it criminal under an anarchy?
Without consent and having to assume that she's not a masochist, torturing a child is aggression, just as if you were torturing an adult.
What profit motive would a private security firm have for taking this case, the parents didn't have anything to pay restitution with; and what they did have was already under due claim because of the theft that led to this discovery.
They would be forced to pay restitution out of whatever they earned in the future and they would be kept track of until they paid it off. Anyways, this is getting more into implementation which is really an entrepreneurial decision. Imagine if shoes had always been made by the government and I suggested we privatize it. You could fire off a bunch of questions such as:
Would everyone make their own shoes?
If not who would make the shoes?
How many would be made?
What colors would they be?
What would they be made out of?
How much would they cost?
...
..
.
The answers that I could give wouldn't be exact. I could just tell you that like most things, such as computers, tennis rackets and pizza, they could be made better by people that were motivated by profit and loss rather than motivated by special interests and swaying votes. If I were forced to answer all your questions down to every last detail, nothing could be done privately and everything would be the domain of government.