We study arbitration mechanisms for child custody arrangements in the shadow of a default court ruling that will be made without any consideration of the parents' preferences. We show that the ideal default custody, providing the best chances for the efficiency of arbitration, is not an extreme custody regime which assigns full custody to one of the parents. Instead it is the custody arrangement that maximizes the reservation payoffs of the most difficult parent types to persuade to forfeit this default custody arrangement and accept arbitration. As long as all involved parties' utility functions are concave, this result also generalizes to the multiple-parties cases where custody settlements require the consent of more parties than the two parents. Further, when parents have the quadratic disutility function, efficient arbitration is possible only under the ideal default custody, regardless of the parents' type distributions. This possibility result extends to the case where custody decisions involve more issues/dimensions than the share of the time that the children will spend with each parent. It also extends to the multiple-parties setting. Finally, we identify a class of utility functions that do not allow e¢ cient arbitration even under the ideal default custody.