Ukraine's borders were recognized under international law. It's quite clear the children of Ukraine are being used to swell the lagging population of Russia. These children are sent to re-education camps [2], where they are taught to deny the legitimacy of the Ukrainian state.
"The Parliamentary Assembly of the Council of Europe (PACE) has recognised the deportation and forcible transfers of Ukrainian children to the territory of the Russian Federation as genocide." [3]
[2] https://www.theguardian.com/world/2023/feb/15/ukraine-childr...