Dear A.:
I would not force her. Did the muslim part really scare her or was it perhaps a veil (which to a 4-year-old may look like a mask)? Or perhaps the dad had a big nose or something else that scared her - totally unrelated. Perhaps it was uneasiness of not knowing the language, if they did not speak English. Could have been a lot of reasons, and every person should have the right to dislike individual people or small groups, much as I dislike certain cuisines just because they do not appeal to my taste. If in her case it was the clothes or a unibrow, it would have little or nothing to do with them being muslims and I would not worry. See how she acts around muslims in more casual clothes who look more familiar. Get a sample size of more than '1'.
Here's a second thought though: After 9/11, speeches of hatred and seeing the US flag burned in Middle Eastern countries, are Americans really obliged to treat all people equally? Legally yes, but if I was an American, I would certainly dislike people who have no respect for my flag and what it stands for more than those who don't. As far as I know, national socialism, communism, Christianity and Muslimism (?) have the stated objective to spread and take over. You cannot peacefully coexist with those who have such mission statements. A black person next to you may have a loud boom box, but otherwise does not do anything to affect your life. A KKK person next to a black person is likely to do something to that extent, so a healthy prejudice based on mission statements and observed behavior should be in order.
I have personally two modes: Individual and collective. As individuals I give people the benefit of the doubt and see how they interact with me. As a collective, I shy away from known sources of trouble. I have not read much about Native Americans robbing 7/11s, but if I see guys with gang bandanas around their heads in a store, I keep driving to the next one.
I commend what you are teaching, but I also suggest to let her develop a personal guard and not force her in the name of political correctness. If this persists, talk to her.
Good luck,
W.