In this day and age, I don't like my wife having to leave our baby in the car while she returns the shopping trolley so I made her this removable shopping trolley token. It seems to work on Woolies, Coles, Big W and other supermarkets with similar coin operated locks in Australia.
Download the STL files, print them out, and thread them on your keyring. There are two sizes - one to immitate a $1 coin (larger) and one to immitate a $2 coin.
Insert into trolley lock; the chain should pop out. Rotate the token and it should slide straight out. Now you can ensure your children are safe because you don't have to leave your car to return the trolley.
I've included the OpenSCAD source files if you want to modify the design.