My toddler can beat up your six-year-old

So, we're at the library, and my 20-month-old is checking out a glassed-in display of origami insects.

A little girl comes up behind him and says something like, "Those aren't real bugs, they're only made of paper," in that irritating kind of I-know-something-you-don't-know voice.

Blue-Eyed Boy turns around and give her a look that clearly says, "And who asked you, anyway?" Then he puts up both arms to ensure she is no longer in his immediate body space. He barely touches her, but she flinches.

The girl, who is somewhere around 5 or 6 years old, turns around to her mother and says, "Mommy, he pushed me!" The mother, who saw the action herself, says, "Well, he's just a baby, he needs to learn not to push."

I said nothing. Maybe it was the two-year-old in me, but I just did not see any point in defending a six-year-old against a "baby."

My poor firstborn... I'm sure he was the victim of plenty of incidents where I disciplined mainly to appease the feelings of another parent. I really try to avoid that these days. Do you think I'm wrong? Should I be doing more to encourage kindness and tolerance in my secondborn?