Most of the claims of relocation of Ukrainian children to Russian families come from the government of Ukraine, not from independent investigative journalism. The Ukrainian government has obvious reasons to do this atrocity propaganda. The Russian government itself justified the start of the war on similar grounds, arguing that Russian ethnics in eastern Ukraine were being genocided by Ukraine through forced Ukrainization and the banning of Russian language. I don't take the Russian government seriously when it makes such claims because I haven't seen much independent journalism confirming it, I apply the same reasoning to the claim of Ukrainian children being kidnapped en-masse and forcibly Russified.

As far as I know, there are only a handful of independently confirmed cases of Ukrainian children being sent to adoptive families in Russia, and most children evacuated by the Russian military have been returned to their families in Ukraine. There aren't many cases of either because, as you may expect, in an attrition war in which the movements of the frontline are slow, there are very few children near the frontlines.