This was/is standard practice. If you’re adopting out of foster care then the kids will know, generally speaking. If not, it’s entirely up to parents and the professional consensus has been to simply lie to your adopted children about their real parents. This extends as far as forging birth certificates (forget her name but an enterprising social worker/human trafficker working out of Memphis got this started and now it’s standard practice to falsify adoptees’ birth certificates to show the adoptive parents as their birth parents.

I don’t know that kids know that they’re adopted, necessarily, if they’re young enough and being gaslighted, but there tends to be a sense of not being quite like your siblings, or your parents even before you figure out what adoption means.

Fun fact, the USAmerican domestic supply of children is also built entirely on theft and coercion. The private international adoption market operates on suitcases full of cash, but the domestic one involves a series of financial incentives to steal kids and put them in adoptive homes. There is no corresponding set of financial incentives to do anything about the structural conditions that put parents in the position of having their kids stolen from them. In more or less every case the solution to the conditions that led to the kids being taken is the direct application of fistfuls of cash to the parents themselves. Substandard housing? Go after the landlord, instead.

ACAB includes family cops and the adoption industry.