Agreed they don't take reservations. The Wing is the one close to gates 1-4. The Pier (reopens 1 August) is up near gate 65 (and has daybreak rooms - The Wing has Cabanas, which are different).

Experience reported here and by other friends is that CX can be quite tight on lounge access, although they will normally make an exception for a young child.

Remember that as a Sapphire you only get Business Class lounge access unless flying in F. This means you don't have access to The Cabanas or The Haven (the full restaurant) in The Wing, or to the daybreak rooms in The Pier.