From the Guardian article in the OP
The mother of a 14-year-old trans boy, who asked not to be identified, told the Observer: “Not all trans kids would want to choose it in the same way that not all non-trans children would want to go on and have their own children. But it isn’t realistic to expect a child to want to be able to have their own children, so it’s up to the parents to say ‘we need to preserve that option for them’ so that, when they get to an age when they do want to start thinking about it, at least then they have the choice.”
This parent is admitting that their child does not fully understand the consequences of their actions. And yet this parent has allowed their child to start down this route. Probably arguing that they 'know who they are' and that they do understand what they are doing.