There are various (english) news articles (for example [0][1]) and also a paper for the European commision [2] (so called ESPN Flash Report) claiming that.
But besides those sources there is also the fact that the child-raising benefits are only unconditional from the second child on, for the first child there is a maximum monthly income to get it.[2] If the main goal was the well being part, then this would not make much sense.
[0] http://www.thenews.pl/1/9/Artykul/250612,Poland-pays-out-mil...
[1] http://nytlive.nytimes.com/womenintheworld/2016/02/11/poland...
[2] http://ec.europa.eu/social/BlobServlet?docId=16077&langId=en [PDF]
