Schönbrunn Zoo is the world's oldest continuously operating zoo, founded in 1752 as the Habsburg imperial menagerie. It still sits inside the Schönbrunn palace grounds, which makes it feel very different from a modern edge-of-town zoo.
It is a strong family stop, but not only for children: the baroque setting, historic pavilions, and long institutional history make it part of the Schönbrunn story. Give it separate time rather than treating it as a quick detour after the palace rooms.