In what circumstances will a court dismiss a motion alleging that a juvenile offender was released into the custody of his mother?

In Thompson v. County of Alameda, supra, 27 Cal.3d 741, 167 Cal.Rptr. 70, 614 P.2d 728, a county released a juvenile offender on temporary leave into his mother's custody at [68 Cal.App.4th 1463] her home. The juvenile offender subsequently killed a five-year-old neighbor. Plaintiffs alleged, among other claims, that the County negligently chose the juvenile's mother as custodian, and failed to supervise the mother's activities. (Id. at p. 748, 167 Cal.Rptr. 70, 614 P.2d 728.) The trial court dismissed the case after sustaining a demurrer without leave to amend.