The original BBC article is here. The basic principle of liberty is to allow others to do what they want provided it does no harm to others. Under this principle, ignoring her age, we should let her do it.
Why would we not let her? Because we think she’s not capable of thinking through her decisions yet, and so her decisions should not be respected. Normally it would be enough to simply let the parent delimit a child’s decisions using their guidance.
In this case, however, the parents are supposedly not properly delimiting the decisions of the child. So someone else must step in. I think the case for overruling parents’ authority over their children is valid in principle – imagine a parent who has convinced their child to kill themselves “voluntarily”, or some other sufficiently odious behaviour.
So where and how do we draw the line? Let’s consider the general case of a child who “wants” to do a dangerous activity (that would cause no harm to others). The parents support the child. Should the state intervene?
We need to consider:
1. The state of development of the child (a 6-year-old’s decisions should be respected less than a 13-year-old’s.)
2. The extent to which the parents are influencing the decisions of the child (if the parent is very manipulative or if they are letting the child make their own decisions)
3. The danger to the child (crossing the street alone vs. playing in a volcano alone)
We would have to bring these three variables together and decide subjectively if we think it crosses the line. In the sailing case I have the first two variables well measured but I don’t fully understand the magnitude of the third. Perhaps there is a way to mitigate the danger? With GPS or with the parents following closely behind?
My final judgement is that if it’s not too dangerous, the state should not intervene.