Hola Passant

In this sentence from our reading exercise: "Los adultos compran este calendario para los niños", "para" is used (not por) because we are talking about someone being the recipient of something. The children in this case are receiving the calendar and are the ones benefiting from this. 

There is a specific lesson explaining this. Have a look here. 

Bear in mind that we have lots of different lesson about using por and using para, as it is always confusing for students -we only have one in English: "for", but have two in Spanish depending on what the precise meaning is. 

If you want to see or keep these lessons in your notebook, here is a link where you'll find pretty much all the lesson for por and para that we have in Kwiziq.

I hope it helps.

Saludos

Inma