There's a notion that the yetzer tov enters the body at the age of bar mitzvah. If this is so how do we see children that behave properly and act kindly towards others?
Rabbi Simcha Wasserman zt'l said (not exact quote. see "Reb Simcha Speaks"): "children are like new immigrants which arrive at a new country. The immigrants quickly adopt the customs of the local population. So too children quickly adopt the behavior of their parents..."
this answer does not explain all cases but i think it explains many.
This question is so strong that I think it is reason enough to re-read and re-interpret the original Chazal source of that statement.
יצר הרע כיצד אמרו שלש עשרה שנה גדול יצר הרע מיצר טוב ממעי אמו של אדם היה גדל ובא עמו והתחיל מחלל שבתות אין ממחה בידו [הורג נפשות אין ממחה בידו הולך לדבר עבירה אין ממחה בידו] לאחר י״ג שנה נולד יצר טוב כיון שמחלל שבתות א״ל ריקה הרי הוא אומר (שמות ל״א:י״ד) מחלליה מות יומת.
The Evil Urge. How so? They say that for the first thirteen years [of a person’s life] the Evil Urge is greater than the Good Urge. There in his mother’s womb, a person’s Evil Urge grows with him. [After he emerges into the world,] he starts breaking the Sabbath, and nothing is there to stop him; [killing people, and nothing is there to stop him; going out to sin, and nothing is there to stop him.] After thirteen years, the Good Urge is born. Then when he breaks the Sabbath, it says to him: Empty one! Isn’t it written (Exodus 31:14), “One who breaks it will surely die”?
Over there it speaks about the remorse that a person feels after committing a wrongdoing. This specific feeling (guilty conscience) appears only after the age of 13. This is referred to as "yetzer hatov" in that specific context.
Before this age, the child still might feel bad after committing a wrongdoing, but that might be only because afraid of the parents who will be upset.
(Note that the general weakness of my answer, as well as the two other answers, as well all this type of philosophy is that how is it ever possible to honestly distinguish between "real" good/bad deed and "acting as if". Therefore, all these suggestions remains very hypothetic. Because who are we to judge a whole group of people (children / non-Jews) and assess the honesty of all of their actions? Nevertheless, I think my answer at least has to do more with the text.)