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.