I would not suggest taking matters into your own hands and using self-help methods. You should go through the court and request a change of custody. I strongly suggest that you contact local counsel to assist you with this process. Good luck.
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I agree with Mr. Lewis and want to add that if the case is out of California, you will need a California attorney to file for modification. You should act quickly, before mom wanders back into their lives with her boyfriend.
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If you are the children's father then you can pick the kids up on your next scheduled visitation. If you feel the children have been abandoned by their mother then you will need to file with the court who has jurisdiction. Speak to an attorney immediately to find out your options or the grandparents may have standing to fight you for custody.