If your child is above age 14 and he or she wants to play at an elite level, it may make sense to have your son or daughter concentrate on one sport. While there are some athletes who can excel at several sports, it appears that, as in other fields, we are now in an age of specilialization when it comes to sports. It is particularly hard to play two sports if their seasons overlap.
If your child is playing at a recreational level, it is fine and probably wise to keep him or her in several different sports.
It should be noted that this is a delicate issue which is different for every kid and it may be an issue that parents, coaches and kids need to revisit and reevalute from time to time.