Friends Come to the Door (and Maybe a Stranger or Two)

I'm not sure of the purpose of this 1960 pamphlet published by United Church Press. It seems to encourage children as young as 3 to open the door to whoever knocks.

 If that bottom panel is glass, it would appear friend Charlie has arrived in some sort of alien bean pod.

The way Charlie acts, going straight to play without even acknowledging his friend lends credence to my "Body Snatchers" theory.

"Isn't it fine to have a door and a bell For mailmen, delivery men, and, well--" Any potentially dangerous vagrant!