MISSOULA, Mont.—During a short recess in their first hearing, the young plaintiffs in the Lighthiser v. Trump lawsuit gathered outside the courtroom to dance along to a TikTok video.

They’d been listening to nearly two days’ worth of testimony and legal arguments that began Sept. 16, sitting attentively in their button-down shirts, khakis and one thrift-store purple suit. Five of them had taken the stand themselves, describing how climate change was threatening their futures, then facing cross-examination from U.S. Department of Justice lawyers. It was time to just be kids for a moment.

The 22 youth plaintiffs had their first day in court at a hearing before the U.S. District Court of Montana in Missoula, where they, supported by a slate of expert witnesses, asked Judge Dana L. Christens

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