Trying to maintain German (minority language) at home

When did our home language switch from the target (minority) language to the community (majority) language?

This is a good question with a long and somewhat complicated answer. Since we've raised our children in two batches, we have two entirely different scenarios. Currently, our second batch of kids (who at the time of this post are ages 6, 5, and 5) still speak almost exclusively in German to each other. But our older children had started switching from German to English (when speaking to each other) by the time they were the same age.  In this post, I'll address the first batch of kids.

Before my oldest son, Ben, entered Kindergarten, he and his twin sisters (2 years younger) mostly spoke German to each other. At that time, their German was much better than their English. They understood English and used it on occasion. However, they usually preferred to talk to each other in German. When they spoke English, it was with a German accent and they often used German syntax: as in "I vont zat not" (I don't want that). They almost always used German with us and we almost always spoke German to them. When we visited non-German speaking relatives, the kids had a hard time communicating with aunts and uncles in English. 

All that changed when Ben entered Kindergarten. When he came home from Kindergarten, he didn't have the German words to talk about his experiences, so he switched to English. I think he also thought it was a little more "cool" to speak English. Once Ben started speaking English to his siblings, they all started speaking English to each other. However, we still spoke plenty of German at home. Karl and I continued to speak only German to them and they usually answered in German. And I would often remind them to speak German to each other and usually, with a little reminder, they would try and switch. They actually never went through a time when they refused to speak German. If I reminded them to speak German, they were happy to switch until they ran across a topic, a word or a situation where English was easier, at which point they would slip back into English. 

In this video, the kids are playing in the garage (which was converted to a play room). Michaela (age 4) is playing with her German friend, Sonja. Ben (age 6) is playing in the background and wants to join in the game. Michaela is mostly conversing in German with Sonja. She has to ask me about a few words like "beach" (Strand). Ben keeps interrupting Michaela and Sonya in English, but switches to German when I remind him. He doesn't mind switching, and does it willingly, he just needs a lot of reminders. When he starts a sentence in English, I quickly interrupt and give him a few German words, and that is usually enough to help him continue in German. And sweet little Kiana (age 4) is just playing on her own in the background. At the end of the video, she rides her horse to go on a shopping trip, 'very far away', to buy honey and bread.