The nouns 'insects' and 'children', though correct grammatically at 'A' and 'C', respectively, are not correct contextually at their positions, because insects cannot understand the lifecycles of butterflies and 'children' are not born from 'eggs'. Also, word 'B' [verb 'discussed'] is apt at its position because it is preceded by subject pronoun 'we' which is followed by a verb to express an action. Thus, if we interchange words 'A' and 'C', then the sentence becomes grammatically and contextually right.
The correct order is CBA. Hence, the correct answer is option (a).
Correct statement after corrections: My children and I looked at the lifecycle of butterflies and frogs last summer, and we discussed about how, when insects die, another one is born from the eggs.