Hi DebinAustria! I was once in a similar situation to your kids (native English speaker, moved to German speaking country/school for several years before returning home age 10 ish).
All I can say is don't underestimate how quickly they will lose the ability to speak the second language if you don't very actively keep it up. We moved back to the UK at the start of the summer holidays, and by the time the schools (and German classes) started in September, and a lot was already forgotten.
One of my siblings specifically went to a high school that taught German GCSE on our return, but tbh this didn't really help as it wasn't the sort of German that you'd speak normally, but much more formal etc (and not really aimed at the right level). So I'd say don't pick a school based solely on that, but instead look for a native speaker class or group outside the school.
Failing that, perhaps you could encourage your DC to speak German to each other when at home, or (if you can speak it too), everyone speaks German at the weekend for eg.
Do they have friends in German that they could set up email with? That would help keep up their writing skills and keep them in touch with native speakers of their own age.
Good luck, I really wish that we'd made more of an effort to keep up our once fluent German, as i've had to re-learn as an adult and it was bloody hard!!