Severity of vows, swears, and promises:

One is not to be accustomed in making vows. One who makes a vow, even if he fulfills it, is called a Rasha and a sinner.[1] One who makes a vow is considered like one who built a Bama at a time that building Bamos are forbidden. One who fulfills the vow is considered as if he has brought a sacrifice on a Bama altar [and is liable for Shechutei Chutz], as it is better to revoke the vow through a sage than to fulfill it.[2] One is to avoid swearing even regarding truthful matters. Yanai the king had 1000 cities destroyed due to swearing even though he fulfilled his word.[3] One who is not careful regarding fulfilling his vows can cause r”l death to his wife [and young children].[4]

Educating children to avoid making vows and swearing:[5] One is to especially pay attention to his young children, that they are not accustomed to make promises and make swears and vows, and is to educate them appropriately with disciplinary action to prevent them from doing so. This is an obligation upon all parents and teachers to educate their children and students not to swear as do the children of idol worshipers.

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[1] Michaber Yoreh Deah 203:1; Nedarim 20a; 22a; 77b

[2] Michaber Yoreh Deah 203:3; Nedarim 22a

[3] Admur 156:2; Michaber 156:1; Tur 156:1; Tanchuma and Midrash Raba Parshas Mastos 22a

[4] Shabbos 32b; Zevachim 29b; Rosh Hashanah 6a; Alef Hamagen 581:102 based on Zohar Pekudei includes young children

[5] Kitzur SHU”A 67:9; 165:1; Rambam Shavuos 12:8