An inverted sentence happens when we reverse (invert) the normal word order of a structure, most commonly the subject-verb word order.
Option A is correct: The sentence has the reverse or inverted word order- Adverb + auxiliary verb (should) + subject (children) + main verb (play) + object + clause. 
Option B is incorrect: The sentence has the normal word order, subject (I) + main verb ( understood )  + object.
Option C is incorrect: The words are not altered as: verb + subject + object, but instead have been altered this way- verb + object + subject.
Option D is incorrect: It's irrelevant.