How to assess

When I want to explain the green plant lesson to my grade two students, first I want them to master the knowledge that green plants need water, air and sunlight to grow up and live. When I am pretty sure that they have mastered the knowledge, I will start practicing the reasoning with them. So I can for example give them a question made of two cases, case A where all the needs of the plant are present and the plant will live and grow up, and case B where one of the needs are absent, deprived from water for example and of course it won’t live. Then I ask them why the plant lived in case A but didn’t in case B, and so on so far. When I have practiced the reasoning enough with my students, then I can give them a reasoning question but I should leave them alone at assessment time to see if they can combine the two successfully (knowledge + reasoning), because if I help them also during the assessment, then I won’t be able to know what do they have learned and what are they able to do alone, and then the result of the assignment will mean nothing since I am not tracking the target.