The three-week period between Shiba Asar BeTammuz and Tisha BAb is an ominous time when there is a greater risk of harm. The Sages speak of a "demon" called "Keteb Meriri" that threatens during this period, and Halacha therefore requires taking certain special precautions throughout the Three Weeks. Firstly, one should not hit a child, student, or, for that matter, anybody, during this period, even lightly. (It goes without saying that hitting to inflict pain is always strictly forbidden.) The Midrash tells the story of a person who was pursuing somebody else to hit him, and Rabbi Abhu stopped him, noting the danger that hitting would pose. The man said that he had intended to give only a light tap, but Rabbi Abhu informed him that there was a "demon" behind him which would have killed the other person if the pursuer had hit him, even gently. This demonstrates the danger involved in hitting, even gently, during the period of the Three Weeks.
For the same reason, one should refrain from unnecessary travel during the Three Weeks. Hacham Ovadia Yosef writes that the rule during this period is "Ashreh Yoshebeh Betecha" "Fortunate are those who dwell in Your home," meaning, it is far preferable to remain at home rather than travel. Children, especially, must be supervised very carefully during the Three Weeks so they are protected from danger. Although it is customary to allow recreational trips and swimming during this period, extra care must be taken to ensure safety. In particular, one must not travel alone, in a deserted area, between the fourth and ninth hours of the day approximately 9:30am-3:30pm during the Three Weeks, because of the danger posed by the "demons" which roam freely at that time. This refers only to deserted area; it is certainly permissible to travel to a city alone during these hours.
Summary: Extra caution must be taken to avoid danger during the Three Weeks. In particular, one should not hit anybody including children and students even gently, and one should not travel unnecessarily. It is especially important not to travel to deserted areas between the hours of 9:30am-3:30pm during the Three Weeks.
