As a general rule, California wants to grant as much parenting time to both parents as possible. Both parents have a number of rights, as well as responsibilities, regarding the care, custody, parenting, and provision of your children. So absent some sort of egregious behavior, one parent cannot unilaterally keep the children away from another parent.
With the facts presented above, there may be jurisdictional issues between California law and Michigan law to look into. Maybe even issues involving child abduction. Either way, a competent attorney would need to know more facts and be well versed in the jurisdictional laws concerning the children. Those laws are referred to as the UCCJEA (Uniform Child Custody Jurisdiction and Enforcement Act).
If the children have been in Michigan for two years, then Michigan should be the home state of the child and the state to determine custody rights, You should contact a lawyer in Michigan and file an action for custody and return of the children. If she files here, you may also file in Michigan and the courts will discuss with one another which state should control. I would guess that would be Michigan. By the way, just because your are the Dad should not prejudice you.