Historically, it's a pretty common way to wipe out a culture/people. Take the children of the unwanted culture and place them in homes of the desired culture to assimilate them and ultimately purge remnants of the unwanted culture. Canada/US/Australia did it to indigenous people, and Soviet Russia did it to a lot of cultural groups. They would also commonly execute local community leaders and replace them with Russians to solidify their hold on the territories they gained.

Russia is just trying to assimilate Ukrainians into Russia and make it really, really difficult to ever disentangle them for generations to come.

They announce this kind of thing for morale. It's important for Ukrainians to know that their government is doing something about the missing children, and has seen a measure of success.

You'll notice that this article is also written in English. Ukraine needs to announce this to an international audience to keep the war in international news and keep international sympathies with Ukraine. News like this is very helpful for them from a foreign policy perspective, so spreading it around without explaining how they saved the kids gets the morale/foreign policy boost without risking their sources.