I agree that it's an interesting problem, and one that I hadn't thought about before. On the one hand, there's the state's obvious interest in controlling juvenile delinquency, an interest that's reflected by (amongst other statutes) Section 602 of the Welfare and Institutions Code, whch grants state court the power to declare any juvenile who "violates any law . . . of the United States" a ward of the court. On the other hand, Congress has granted federal courts "original jurisdiction, exclusive of the courts of the States, of all offenses against the laws of the United States" in 18 U.S.C. § 3231. Does the latter preclude the former?
While it's an interesting point, I think that Justice Werdegar (and there rest of the Court) are indisputably right that the answer is "No." In my mind, to use an analogy different from that used by Justice Kennard, it's similar to Merrell Dow in the civil context. Just like federal law violations can be used to, for example, support a state law negligence per se (or unfair competition) claim without creating federal jurisdiction, so too can state law permissibly regulate juvenile federal offenses as long as this doesn't constitute what we'd normally refer to as the adjudication of federal criminal offense.
Still, a neat issue.