I agree that Jewish children should not wear costumes with crosses or other goyish religious images. (That includes the yin/yang on the ninja costumes)
However, I think that some misconceptions about history are fueling some of the feelings expressed here.
First of all, most of the Crusader massacres were not carried out by knights. They were carried out by ignorant peasants who were in a religious frenzy. Many knights (who were also noblemen) and Church officials tried to stop them. They were so zealous and undisciplined that they wouldn’t listen to anyone.
Secondly, most medieval soldiers were not knights. Knights were all noblemen. They were educated, well-trained and disciplined. In other words, they were unlikely to go off and massacre a lot of people who had nothing to do with their military objectives. You can tell this by the fact that there were a lot of massacres during the First Crusade (when the common people went) and hardly any during the subsequent ones (when the armies consisted of mostly knights and professional soldiers).
Thirdly, most knights were not in the service of the Church. Most weren’t. I think that some of you are referring to Templars. They wore the white tunics with the big red crosses and are what comes to a lot of people’s minds when they think of knighs. They were one of just a couple of monastic orders of knights (they were fighting monks). Most knights were loyal to a nobeman who outranked them. Whether or not a knight was good or bad largely depended upon under which nobleman he served. Some nobelmen were more tolerant than others of Jews and some had different standards of behavior than others for their knights.
Also, if we want to avoid the cosumes of people who have persecuted us, we won’t have many to choose from. People from all walks of life from many different cultures have persecuted us.