Which model Siemens washer do you have? For the WM16Y792GB, the child lock is toggled on and off by pressing and holding 2 buttons that are below the LCD display screen on the left side of the screen for 3+ seconds. The phrase "Child Lock 3 seconds" is just below the buttons. The temperature and spin buttons are above the display in the corresponding location.
For the WM12Q391GB, the child lock is toggled on and off by holding the Start button for about 5 seconds. On this model, the Start button is the one to the farthest right. This button doesn't seem to have a symbol or text indicating that it also controls the Child Lock feature. The manual does tell how to toggle the Child Lock feature on and off.
Many Siemens appliance manuals are available from their web site. Some are available here: http://www.siemens-home.com/ie/customer-service/manuals
. Otherwise, try to find the product site and click on the Downloads tab. The washers listed above both have the manuals on their pages. The WM12Q391GB page is http://www.siemens-home.com/ie/productlist/laundry/washing-machines/WM12Q391GB?breadcrumb=frontloaderwashingmachines
. The WM16Y792GB page is http://www.siemens-home.com/ie/productlist/laundry/washing-machines/WM16Y792GB?breadcrumb=frontloaderwashingmachines
I hope this helps. Otherwise, please add a comment with the model of your washer. I will attempt to locate the Child Lock information you need,