So that summer night Sean's friends convinced Claire to let them drug the children - all of the children - so they could party uninterrupted. That night Claire discovered that Sean was having an affair with their interior decorator. Claire leaves the children behind and heads back to London. She was so angry and upset she figured Sean could take care of their children for once.
The next morning those still there discovered that CoCo had died from the drug. They decided to hide her death by faking a kidnap. Claire never knew the truth.
The epilogue made it clear that the daughter of one of Sean's friends (and future fourth wife) actually gave the girls more of the drugs including to kill both of the little girls. She thought with them gone, he would have no reason to stay with Claire and fall in love with her. Even though she was just a teenager at the time.
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