Russian leader Vladimir Putin signed a decree that will facilitate granting Russian citizenship to deported Ukrainian children, which, in turn, will allow for adopting them as Russians and changing their identity, Ombudsman Dmytro Lubinets said on Jan. 5.
Fourth Geneva Convention:
https://www.ohchr.org/en/instruments-mechanisms/instruments/geneva-convention-relative-protection-civilian-persons-time-war
Then:
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And on to the very next article in the Fourth Geneva Convention:
https://www.icrc.org/en/doc/resources/documents/article/other/57jmat.htm
Also, just to address the “this isn’t an occupied territory, but part of Russia because Russia annexed it, so the Fourth Geneva Convention doesn’t apply” argument that I imagine is Russia’s position: