California Code, Family Code - FAM § 3028

(a) The court may order financial compensation for periods when a parent fails to assume the caretaker responsibility or when a parent has been thwarted by the other parent when attempting to exercise custody or visitation rights contemplated by a custody or visitation order, including, but not limited to, an order for joint physical custody, or by a written or oral agreement between the parents.

(b) The compensation shall be limited to (1) the reasonable expenses incurred for or on behalf of a child, resulting from the other parent's failure to assume caretaker responsibility or (2) the reasonable expenses incurred by a parent for or on behalf of a child, resulting from the other parent's thwarting of the parent's efforts to exercise custody or visitation rights. The expenses may include the value of caretaker services but are not limited to the cost of services provided by a third party during the relevant period.

(c) The compensation may be requested by noticed motion or an order to show cause, which shall allege, under penalty of perjury, (1) a minimum of one hundred dollars ($100) of expenses incurred or (2) at least three occurrences of failure to exercise custody or visitation rights or (3) at least three occurrences of the thwarting of efforts to exercise custody or visitation rights within the six months before filing of the motion or order.

(d) Attorney's fees shall be awarded to the prevailing party upon a showing of the nonprevailing party's ability to pay as required by Section 270.