The final part of the Three Weeks culminate in the special fast day called “Tisha B’Av”.
1. Like 17 Tammuz, 9 of Av is a fast day, and fasting applies to both men and women who are physically capable of fasting. Children below the age of bar/bas mitzvah are not required to fast on any fast-day, with the exception of Yom Kippur. 
2. There is one restriction which applies to children who have reached the age of understanding on all fast days. Parents should only feed simple foods to these children on these fast days. Delicacies or specially tasty things should be avoided. In this way, the child is educated to take part in the fasting of the adult community. No such restriction applies to children too young to understand what a fast day is. 
3. Some have a custom, nevertheless, to train children who have reached the age of chinuch to fast part of the day if possible. 
1. On 9 Av, it is forbidden to learn Torah except for mournful or sad subjects, because learning Torah makes one happy. Similarly, parents are bound by the mitzvah of Chinuch to train older children who can study on their own NOT to study Torah on Tisha B’Av, and parents should refrain from teaching Torah to smaller children on Tisha B’Av as well. 
2. There is a machlokes as to whether parents may teach children the same subject matter that adults may study (Iyov, Eichav, Yirmiyahu, etc.) on Tisha B’Av.   Magen Avraham forbids this since teaching Torah in any form is prohibited on Tisha B’Av. Other poskim permits this. Nevertheless, Magen Avraham does not prohibit parents to give their children a simple narration of the events associated with the destruction of the Beis HaMikdash. 
WEARING LEATHER SHOES
1. On Tisha B’av, adults are forbidden to wear leather shoes. There is again machlokes as to whether parents are bound to train children who have reached the age of understanding to observe this.
Chochmat Adam’s opinion is that it is not required because parents are not obligated by the duty of chinuch to do something which results in discomfort for their child. However, Rabbi Moshe Feinstein ruled otherwise. 
2. Children who have reached the age of understanding should be trained to wear non-leather shoes on Tisha B’Av. The present custom though is that even children below that age do not wear leather shoes. 
1. Rabbi Moshe Feinstein ruled that parents are required by the mitzvah of chinuch that their children (above the age of chinuch) observe all the restrictions of Tisha B’Av. 
HAVDALAH WHEN TISHA B’AV FALLS ON MOTZAEI SHABBOS
Some poskim rule that a minor who reached the age of chinuch in the mitzvah of havdalah should recite havdalah (only Hamavdil and Boray Pri Hagafen) prior to eating on Tisha B’Av on Sunday.
This concludes our 3-part laws of Chinuch concerning the 3 weeks.
 A Parent’s Guide to Teaching Children Mitzvot by Rabbi Shmuel Singer
 Children In Halachah by Rabbi Simcha Bunim Cohen.