Explanation of The Book of Divorce By Shaykh Muhammad bin Salih al-Uthaymin Book 3

Thus, the presence of a third person negates their being alone, but this third person must be sensible and vigilant enough so his presence can be a guard against temptation, or else evil would be done while he is unaware. This is why the third person cannot be a little child, even if he has developed some perception [i.e. as of seven years of age] as this is not enough to negate the meaning of "alone with her" unless he is an exceptionally smart child who is keenly aware of what goes on around him. Otherwise, a seven year old child is not enough, for the man can give him anything to play or stay busy with, and then he can say or do whatever he wants. Thus, there must be a third person in the company whose presence prevents evil.