Perhaps I should read the owners manual. I stand corrected, the switch between the window switches is to lock out the rear windows.
The slot to enable the rear child safety door locks is similar to my Honda Accord except that Honda provides a lever to actuate the child safety door lock where with Mercedes you must use a tool to actuate the child safety door lock. Mercedes method does have one advantage: the child can't deactivate the safety door lock themselves. When my kids were small they figured out that by sliding the small lever that they could open the door themselves and they would on occasion slide the lever as they were getting into the car. Mercedes method does have one disadvantage: they do not mention the child safety door lock in the manual.
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles