particles - How to break down who did what in AにBをCさせる

I'm trying to understand the following:

後輩にサイフを開かせることはないよ

I guess this could be generalized as AにBをCさせる.

It's confusing. Maybe if someone can break down how to understand this and then I can memorize it.

Answer

Usually, you have a sentence like:

後輩がサイフを開く
koohai ga saihu wo hirak-u

'the junior opens the wallet'

where the subject takes が and the object takes を.

There is a morpheme -(s)ase- 'let', 'make' that expresses causative. The way you use it is that you embed the sentence, and change the embedded subject が into に, take another subject that will be the causer.

君が後輩にサイフを開かせる
kimi ga [koohai ni saihu wo hirak]-ase-ru
'you let [the junior open the wallet]'

In your example, the subject 君が is actually omitted, so you have

後輩にサイフを開かせる