Timing and stubbornness is also going to play a role. Growing up as a bilingual kid, my brother and I rebelled at a certain age because we did not want to have to learn both if English would be the one that was going to be used. I suppose we did not have the potential benefits explained but as kids we probably wouldn't have cared.

That said, my Chinese is still pretty good, but the real reason for that is immersion. There were times I had prolonged opportunities to speak and listen, e.g. visiting family that did not know English. That's probably the easiest way to get the second language. On a need basis.

But it's still possible at home if you do not give in to the kid and insist on using the mother tongue. In practice, this happened too, and the result is that our listening in the language is pretty good, but not the speaking part. Listen in Chinese, speak in English.

I do agree about the benefits and there's all sorts of cultural things embedded into the language that
is interesting.

Viele danke :) (I think?)