Yes, definitely 100% yes put your kids in boxing and martial arts regardless of gender to strengthen their sense of morality, self esteem and most importantly so that they can protect themselves against bullies and sexual predators! The perfect age to start them at is when they are potty trained, can talk in complete sentences, and can read and write. That age is typically at age 4-5. Before you get scared about your kids getting hurt in a contact sport you should be aware that professional instructors will not allow kids to get hurt and will mostly teach them to strike targets, teach them the fundamentals of discipline and perfect their form. Kids will eventually competitively spar one another when they reach the age of 6-8 and even then they aren't in any real danger since good instructors will already have indoctrinated them so that they will have respect for each other and that they aren't allowed to use more than 30% of their actual strength. Females should be in boxing and martial arts so that they can defend themselves when it comes to sexual predators and kidnappers, females have a really high rate of being targeted so it is highly suggested that they know some type of self defense.