The real secret, I think, is constant exposure. When we previously lived in Germany, I never spoke German with the children. They knew I understood it, so they would never say a word in English to me, but I simply spoke English to them all the time. And Rose and I always spoke English together - I didn't actually speak any German at all until we'd been together for a few years, so we have always been more comfortable speaking English together in any case. When my parents came to visit or we made a trip to America, it was clear that it was working; as soon as they encountered people who didn't do what they wanted when it was demanded in German, the kids would switch it up and make their requirements known in beautiful, unaccented American English.
We obeyed roughly the same rule during our five years in America, with the exception that I would speak a lot more German with the kids, mostly because speaking German at home was how I was trying to keep in practice. But as I said, the kids were not enthusiastic participants.
Now, though, Elijah, the middlest of our three children, is sitting on the couch with his cousin, teaching him to play The Legend of Zelda: Breath of the Wild, in perfectly beautiful German. Considering that he's been speaking German again for all of two days (we spent the first nine days, remember, in the bubble of quarantine), his ability is incredible. He's making lots of grammatical mistakes, especially confusing the gender of nouns and conjugating irregular verbs incorrectly, and he's missing a few words - teleport, goal, climb - but his cousin is infinitely patient and in the time I've been typing this I've heard Elijah use all three of those words correctly. He'll be indistinguishable from any other native speaker within a couple of months.
It makes me so happy to see my children growing up bilingual. I was such a poor student, in such a bad educational system, and had such awful teachers, that I literally never realized that I had a knack for languages and a passion for learning them until I started learning Japanese as a young adult. I'm really jealous of them for getting a second language right out of the gate. And, of course, since they are now going to be getting their educations in Germany, they'll have the opportunity to pick up another two languages before heading off to college.
Of course, I am very aware of the danger of projecting my own passions onto my children. I'm not going to be some super pushy Polyglot Papa, forcing the children to their language lessons every day and making sure their Latin is up to snuff. It's entirely possible their passions are going to be electronic music and football, and if that's the case I'll dig deep and find a way to love them anyway. But whether they come to love languages as I do or not, they'll always have the gift of being conversant in two languages and comfortable in two worlds. I think that's pretty special.