It's not that simple. Kids learn language from their peers just as much as (maybe more than) from their parents.
I'm living in the US: I always speak Korean to my teen kids and they mostly answer in Korean, so they have no problem understanding phrases like "When is dinner today?" However, because they've never been to school in Korea, they basically have zero knowledge of Korean words for, say, "descendant", "ambassador", or "independence", while being totally familiar with their English counterparts.
OP is talking about how if kids learn to be bilingual+ at an early enough age then it makes learning languages that much more easier. This is compared to people who speak one language and try to acquire a second but start in, for example, middle school or later. People in the second boat are at a significant disadvantage. It's a well-studied area and one way of picking up second languages earlier is via peers at an early age as well.
It doesn't mean your points are invalid, but in general people like your kids are just going to be way better at learning language.
IMHO, it's almost cruel that many colleges expect all students to fulfill a 2 year foreign language requirement, even if they have no prior exposure or education. You end up with language learning that is watered down and generally a waste of the learner's time, in addition to making their education more expensive.
> You end up with language learning that is watered down and generally a waste of the learner's time
I agree it might often be a failure, in say, achieving fluency. But certainly no a waste. Exposure to foreign languages, and their cultural contexts, can be very enriching, especially the more distant they are from your own culture and language family. And the contrast with your own language and culture can teach you much about your own.
This is a common argument but it completely ignores any idea of efficiency in learning or learning outcomes that benefit the student.
Most people would get way more out of studying a culture and its context by directly studying that and in way less time. This time saved one could spend as 4 semesters of specialized cultural studies, one full cultural study abroad semester, or learning about other things (comparative linguistics?).
There's also the idea that you can only fully appreciate works of literature in their original language, but learning Russian to appreciate Dostoevsky is overkill and many people love his work who don't know the language. Learning about other cultures in translation is arguably going to be superior since any translation you can do is with skills way below fluency.
I'm living in the US: I always speak Korean to my teen kids and they mostly answer in Korean, so they have no problem understanding phrases like "When is dinner today?" However, because they've never been to school in Korea, they basically have zero knowledge of Korean words for, say, "descendant", "ambassador", or "independence", while being totally familiar with their English counterparts.