For security measure, people use pocket knives, carry on blunt objects, pepper spray and etc.
I got whistles attached to my kids backpack. They use their backpack to carry their ipad, water bottle, sunglasses, drawing material and etc. They take the bag every time they go out. They have been taught to use the whistle when needed.
My son's bag
My son's whistle (bought in Explorer Outfitter, DaMen Mall for RM25 but it is a little cheaper in ACE Hardware, my daughter's (Fox 40 Striker, sports stainless steel whistle, bought in Sabah for RM35) and finally mine, antique Bronze Whistle
Cochlans Camping Whistle, Has a compass, magnifying glass and thermometer (on the reverse)
British Brass Police Whistle. Just like all the other whistles in this post, to get the desired sound, you need to give it a loud blow
Part of my ECD carry when I go out (* note - not all the tools I carry at all time. I rotate the tools I carry