What is the ruling on the appearance of a very small amount of 'awrah (1 or 2 cm of the thigh) due to tears in the trousers?

The minor 'awrah whose exposure does not invalidate the prayer is that which is not considered indecent in the eyes of people. Its determination is referred back to custom and common practice, not a specific measure like "the tip of the pinky finger." What you mentioned in your question is considered minor.