One way around the problem in the store is to find another customer (or two) and ask them to watch the child while you go to the front of the store and tell an employee (or wherever you find an employee.)

If you get other people involved, that's generally better than taking on the responsibility alone.

I don't know if people have seen too many CSI child abduction episodes or if they are just ruder than they used to be or what.

I hope that sometimes people's suspicion actually helps people.