Trying to be brief ... California has absolutely no jurisdiction to determine ANY CUSTODY issue including parenting time (ie visitation). More importantly, Ca. has absolutely no jurisdiction over you or your children or the mother as it relates to the children. Your state, the Home state, has jurisdiction over all parties. Unfortunately, it is common to file false accusations in an attempt to justify or substantiate the filing in a foreign state (ie Ca. in this case; ie ex-parte ... emergency ... when none or EVIDENCE exist even under the most feeble excuses).
You should file for CUSTODY in your state ASAP but definitely before the children have been absent for 6 months. You should gain custody hands down since the other parent is not present to contest your custody. Demand the return of the children. This custody ruling is temporary and may be changed later even though the other parent is known to the system to have "dirty hands".
You should NOT appear in Ca. or respond to any Ca. legal filing for fear of giving Ca. any reason or excuse TO TAKE jurisdiction.
The divorce/custody statutes of a state do not kick-in until a proper jurisdiction is established. At the present, legally, your children are considered to be "kidnapped" and all state and federal statues are highly favorable to the left-behind parent and against the parent that abscounded with the children.
All states have enacted supposely uniformed standards. The foundation act is known as the "Uniform Child Custody Jurisdiction Act" (UCCJA) and is accompanied by federal statues known as the "Parental Kidnapping Prevention Act " (PKPA).
Many may want to consider these situations as mami-pammi with the thinking that the children are OK since they are with a parent. Statistically, children have not fared well when they are jerked from the other parent in this manner and this is nothing new. The UCCJA was initiated in 1968 and the PKPA in 1981.
It might be best to retain counsel that will work with you and with UCCJA/PKPA experience if possible. The jurisdictional presentations should be cheap, unlike custody issues, and are straight forward and leave nothing to the bias discretion of the court. At very least, file pro se' (self) for custody in the Home (your) state.
By the way, your issue would have been better placed on the "Custody" board. An attorney may have talked to you about "visitation" and that is in no way relevant until the children are home. For heaven's sake. They are children ... not horses or cattle or automobiles.
Possession is NOT 99.9 % of the law. !!
Best to the children and you.