My kids only get English at school — now they won't even answer me in it. What actually worked for you?

We're a pretty tangled language situation at home: English at school, Korean at home, Khmer in the neighborhood. I'm fluent in English myself, but honestly I have zero memory of how I actually learned it — no memory of the process, I just know how to speak it. So I can't really model anything for my kids, I'm just guessing.

We've tried Duolingo Kids and some English YouTube shows. Duolingo turned into just a game for them — no real carryover, no actual language sticking. The shows are passive; they watch, but I don't see it show up anywhere else.

The part that's actually bothering me: when I try to talk to them in English directly, they don't want to answer. They understand some of it, not all of it, and mostly just revert to Korean or shrug. My third son will straight up answer "몰라" (I don't know) in Korean when I ask him something in English — not because he doesn't understand, I don't think, just because it's easier to disengage that way.

I don't think apps or shows alone are going to fix this.

For parents who've actually gotten a minority-at-home language to stick for their kids —
what worked?
Was it forcing English-only at certain times, something more social (friends, tutors), or something else entirely?