The Ukrainian authorities have identified over 19,000 Ukrainian children who have been illegally deported to Russia since the beginning of the full-scale invasion in February 2022.

Fourth Geneva Convention:

https://www.ohchr.org/en/instruments-mechanisms/instruments/geneva-convention-relative-protection-civilian-persons-time-war

Article 49

Individual or mass forcible transfers, as well as deportations of protected persons from occupied territory to the territory of the Occupying Power or to that of any other country, occupied or not, are prohibited, regardless of their motive.

Nevertheless, the Occupying Power may undertake total or partial evacuation of a given area if the security of the population or imperative military reasons so demand. Such evacuations may not involve the displacement of protected persons outside the bounds of the occupied territory except when for material reasons it is impossible to avoid such displacement. Persons thus evacuated shall be transferred back to their s as soon as hostilities in the area in question have ceased.

The Occupying Power undertaking such transfers or evacuations shall ensure, to the greatest practicable extent, that proper accommodation is provided to receive the protected persons, that the removals are effected in satisfactory conditions of hygiene, health, safety and nutrition, and that members of the same family are not separated.

The Protecting Power shall be informed of any transfers and evacuations as soon as they have taken place.

The Occupying Power shall not detain protected persons in an area particularly exposed to the dangers of war unless the security of the population or imperative military reasons so demand.

The Occupying Power shall not deport or transfer parts of its own civilian population into the territory it occupies.

Then:

According to Lubinets, Russia aims to grant Russian citizenship to more of the deported Ukrainian children so that they don’t de-jure count as Ukrainians remaining on their territory anymore.

[snip]

“We understand the terrible consequences of such actions because the next step after they receive the ‘citizenship’ of the Russian Federation will be the adoption of children as Russians, and after that - the change of their personal data.”

[snip]

Originally named Marharyta Prokopenko, the child was abducted from a children’s home in then-occupied Kherson, adopted by a Russian lawmaker’s family, granted Russian citizenship, and renamed Marina Mironova. Her birthplace was purportedly changed in the records as Russian Podolsk.

And on to the very next article in the Fourth Geneva Convention:

Article 50

The Occupying Power shall, with the cooperation of the national and local authorities, facilitate the proper working of all institutions devoted to the care and education of children.

The Occupying Power shall take all necessary steps to facilitate the identification of children and the registration of their parentage. It may not, in any case, change their personal status, nor enlist them in formations or organizations subordinate to it.

[snip]

https://www.icrc.org/en/doc/resources/documents/article/other/57jmat.htm

The Fourth Geneva Convention prohibits an Occupying Power from changing a child’s personal status (article 50). Therefore, neither his nationality nor civil status should be changed if his country is occupied during an international armed conflict.

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:

Article 47

Protected persons who are in occupied territory shall not be deprived, in any case or in any manner whatsoever, of the benefits of the present Convention by any change introduced, as the result of the occupation of a territory, into the institutions or government of the said territory, nor by any agreement concluded between the authorities of the occupied territories and the Occupying Power, nor by any annexation by the latter of the whole or part of the occupied territory.