For the FBI to "take over" a case, there have to be particular criteria met...such as evidence of a federal interstate crime, murder of a law enforcement official, evidence of interstate drug trafficking, etc. They can't take over a case just because it is serious, because the victims family requested it, or because the local jurisdiction seems inexperienced/is not making progress. What they can do is participate as an investigative partner in a situation involving a missing child “of tender years,” even though there is no known interstate kidnapping aspect. “Tender years” is generally defined as a child 12 years or younger. The FBI will offer assistance from various entities including the FBI Laboratory. But any case would remain the jurisdiction of the local agency unless one of those specific federal crimes were uncovered.