The adults-only area on the upper decks (adult pool, Cove Cafe, Satellite Falls) is adults-only all day.
The Europa area (and its equivalent on other ships) becomes adults-only at night, so kids are usually allowed in areas such as La Piazza and The Tube during the day. However, Skyline and Ooh La La aren’t typically open during the day, so they are de-facto only for adults. As mentioned above, the Daily Navigator app will indicate which venues/activities are open to all and which are only for adults always or after a certain time.