A very simple one... this is how you can activate the child lock for the 5 door ST170.
Before we start, here is the list of tools you need for this job:
Basically, just open the rear door and take a peek inside the lock hatch. You may have to clean it as it could be covered in black guk, like mine! (Click for bigger picture)
A close up photo, if you can't see where it is...
Basically, just turn it anti-clockwise to activate it. Turn it clockwise to disable it, Clock it and try to open it from the inside and you will not be able to open the door.
And that's how you keep your child safe. :)