[replying to the hypothetical "Gypsy A wants to marry Gypsy B, the 12 year old daughter of Gypsy C. They all consent. The whole gypsy ghetto consents. The Spanish Popo don't."]
Before anyone gets their panties in a twist, consider the following.
In order to have a theory of the rights of children, it must be clear what the nature of a child is.
In the context of law and rights, I think it makes the most sense to recognize that it is not physical, but mental development which defines childhood.
For instance, paraplegics like Stephen Hawking are incapable of commanding their body to do certain tasks, but they may still be adults. Although these disabled people lack certain abilities that are standard in most humans, they do not lack the characteristic mark of action, just the ability to wield many means which others take for granted.
So the defining mark of childhood in this context is psychological, as opposed to physiological immaturity.
From this nature of childhood, we can deduce that it is not a one-time switch flip, but it is possible for a given person to move in and out of psychological maturity through the course of their life.
Consider a sleeping man, certainly this man is (temporarily) psychologically immature. This individual is not capable of negotiating for his own care and instead requires others to do so for him.
This is especially relevant in the "what-if" scenario of an unconscious man lying in the snow and freezing to death. I argue that a paramedic taking this unconscious person to a hospital is analogous to a mother carrying her toddler.
This guardianship role taken up by the paramedic/mother is scarce and must be singly held by the homesteader, under libertarian property rights theory. The reason is simple, there can be conflicts over the specifics of how the guardianship is to be performed.
Childhood, to capture the nature of a child as a psychologically immature human, can be thus defined as the state of being incapable of expressing one's own will and the guardian is the person who takes it upon themself to preserve the child until a time that they gain the ability to express their will.
Now, if, in your hypothetical scenario, the 12 year old daughter of gypsy C is someone who is sufficiently developed in order to be psychologically mature, and she consents as well. Then there is no conflict.
The consent of the Spanish popo in this matter would not matter, unless the Spanish popo moves in to act, in which case they are an aggressor, in which case everybody has the right to use force against them.
If, however, the 12 year old daughter of gypsy C is not psychologically mature and therefore not capable of giving valid consent, then what is happening there falls under abuse performed by the guardian.
What is important to note here is that the guardian is not the owner of the child, but rather the owner of the right to protect that child. Any abuse performed by the guardian onto that child implies an abandonment of that right, implying that the guardian must notify interested parties that the child is available for adoption. In other words, the right to protect that child becomes up for homesteading.
Do note that this requirement to notify potential adopters is not a positive obligation, merely a negative obligation to not forestall.
To quote a bit from Ian Hersum (2020),
A Rational Theory of the Rights of Children:
Since a child’s preferences cannot be known, the proper method of raising him is impossible to determine, so his guardian is largely free to engage in any actions that he wishes to in relation to the child, as long as he does not deprive him of his innate function or form. While refusing to feed (or care for in other ways) a child cannot be understood as an act of harm, since the resources required for such care belong to the guardian and not the child, it still constitutes an abandonment of guardianship rights, but cannot carry a penalty other than one for forestalling. Rather, harm in this context can only be rendered by an active (rather than passive) behavior on the part of an adult against a child. This rules out any form of neglect.
There must be a direct causal link between the action and the effects suffered for it to be considered harmful. [... Verifiable] psychological damage suffered by a child, which is directly attributable to an act of torment inflicted on him by an adult, deprives him of his natural mental functioning which is innately his. This also applies to physiological damage, of which verification and attribution is considerably easier. Any scarring, maiming, mutilation, or other disfigurement, which deprives a child of his innate body, and was suffered as a result of actions taken against him by an adult, likewise qualifies as damage.
In other words, if this marriage is merely a transfer of guardianship ownership, then no conflict has happened yet.
If someone tries to consummate the marriage, then that is clearly an invasion of the child's property rights.
Anybody may move in to enforce the child's rights, including the Spanish popo.
