A basic search and seizure case could work here. The kids can understand the policy, and the reasoning on both sides; on the one hand, you argue that somebody shouldn't go free on a technicality, on the other, you argue that law enforcement officials aren't above the law and shouldn't have an incentive to violate the law. And search and seizure cases do reach the Supreme Court.
No fears, I am well aware that a LOT of Supreme Court cases are not appropriate for kids because they dwell on such huge and historied ideas... but I do think some of the ideas proposed here might work very nicely if made into kid friendly form. Honestly, the idea of debate and discussion and trying to see both sides of a debate is among the key aspects of this for me