Interesting isn’t the way how the media encodes these stories reveals in built gender biases.
If this had been a father and his son, the story would almost certainly have been encoded with words like ‘kidnapped’ or ‘abducted’ (do a google search and you will quickly find many references to this).
But because it’s the mother, not only are the words used far less judgemental (missing suggests the possibility that some misfortune has befallen them rather than a contrived act on the mother’s part to subvert the courts), but the general way the media is treating the story is much more sympathetic. The Today programme for instance, was reporting how the mother’s aim of raising the profile of her case and having it reviewed had ‘been achieved’, the premise of that statement being that there is a justifiable need for this.
In similar cases involving the father, the underlying judgement is that the ‘kidnapping’ or ‘abduction’ simply underline the fact that the father is no fit to be the child’s guardian.