The ring is a gift. Your shock is understandable. They should have tried giving advice to my father!!!
The above is general legal and business analysis. It is not "legal advise" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
I'm so sorry for your predicament. It makes one wonder ...
The engagement ring is specifically identified by CA law as a gift to you and neither the kids nor he can require you to give it back.
If you found this response helpful, please let me know by clicking the "Mark as Helpful" button at the bottom of this response. Thank you. Mr. Richardson practices in San Mateo and Santa Clara Counties, and concentrates in non-adversarial dispute resolution as a mediator and collaborative lawyer. The California State Bar Board of Legal Specialization certifies Mr. Richardson as a specialist in California Family Law. He offers no comments or advice with respect to the laws of any state or jurisdiction other than California. The above answer is a general explanation of legal rights and procedures. Mr. Richardson is not your lawyer unless and until you and he have personally met together. This post does not constitute legal advice, and no lawyer client relationship results.