First consider approaching the school with the request. They should be able to provide you with information on their policy regarding crossing guards or at least tell you who is responsible for assigning them. Crossing guard assignments can vary from location to location. In some places the school is responsible for providing the crossing guards, in other locations the school district is tasked with this and in other places the crossing guard is actually provided by the police or local law enforcement department and they work in conjunction with the school/school district to determine where the crossing guards are placed.
If there are other parents that would like a crossing guard at the same intersection you may want to consider working with them to make your request known to the school. Having more people will help support and reinforce the request.