I'm half English/Russian, DH is German.
Our 3yo dd & 21mo twins were/are raised trilingual just like our 5yo, however we now live in Germany away from my dad (who was the main Russian speaker in their daily lives). They skype my dad & brother every few days and they're (DC) spoken to in Russian.
I don't know if they are or not but from the few conversations I've had with them (in Russian) it's like they've gotten sloppy, excluding our 5yo, compared to how well they spoke in Russian back in England.
Can they lose a language? How do I stop it or improve their language while trying not change the main home dynamic (I speak English to them, DH in German)?