Child custody starts when the guardian appointed by the judge or the parents declares that they are willing to accept the child custody.
The parents have appointed the guardian
In that case, the guardian submits a declaration of willingness to the district court. This must be done within 14 days of the notice of appointment being served on the guardian.
A judge has appointed the guardian
In that case, an oral declaration before the judge is sufficient. A handwritten declaration from the guardian is also permitted; this does not need to be submitted to the district court. There is no deadline, but the judge will only appoint a guardian once the guardian has accepted the child custody.