I knew that their primary language would switch to English. Eventually, it just has to. These kids live in an English speaking society and all attend an English speaking school together. They also have siblings and parents whose native language is English and who speak English to each other. The fact, that the three little ones have been speaking only German to each other for this long is actually quite amazing.
My plan is to continue speaking to them in German.I will also continue to encourage them to use German with each other, although I won't get too bent out of shape when they use English. We will continue to watch German movies and read German books. We'll also continue with our part-time German homeschooling. We have many German routines which will not change, even as their primary language changes. I know this because when my older children switched from German to English, the German routines remained, such as bedtime stories in German, family prayers in German, etc.