Translation:The boys and girls sing in German.
Does the adjective have to be repeated? Allen and Greenbough have a couple of examples of an adjective belonging to multiple nouns e.g. multae operae ac laborīs - "of much trouble and toil" and vīta mōrēsque meī - "my life and character"
So maybe something like Puerī germānicī atque puellae cantant would be more natural? I don't really know, my sense for this kind of thing is still underdeveloped.