Do the laws of Yichud apply towards children? May a male and female child who are not siblings be alone together?
- Example: May a mother go shopping if two cousins of the opposite gender are left alone at home, and both are below the age of Mitzvos?
From the letter of the law, children who are below the age of Bar and Bas Mitzvah are not subject to Yichud regulations when they are together with other children of the opposite gender. Thus, a boy below the age of 13, and girl below the age of 12, are not forbidden in Yichud, and may hence be left together alone. This applies even if there are no other children in the vicinity. This applies even if the male is above the age of 9 and the female is above the age of 3. [Nevertheless, practically, it is proper due to Chinuch obligations and common sense precautions, to educate children that have reached the age of Chinuch to not be alone together on a steady basis. Certainly, children should be instructed that they are never allowed to lock the door of their room when they are there with a child of the opposite gender.]
 Rashal Kedushin 22 in name of Smag and implication of Rambam “The Smag states that the Sages only decreed Yichud if the man is 13 years of age and the girl is three…one must thereby conclude that the Yichud prohibition only applies when at least one of the genders is the age of Mitzvos”; Bach Y.D. 22 “And so is the Halacha like the Rambam, that [only] a child who is of age of Mitzvos is forbidden in Yichud with a child above the age of 3 for a female and 9 for a male”; Dvar Halacha 2:8; Shraga Hameir 8:126; Halichos Bas Yisrael 8:2; The Laws of Yichud [Dubov] 1:8
 Shraga Hameir ibid based on Meiri Kedushin 80a “A male and female child should not constantly be together in the same activity or class, in order so they do not become accustomed to be with each other”
The reason: As although the children themselves are permitted from the letter of the law to be together with each other, nevertheless, this is no different than any other Mitzvah of which we are obligated to educate our children once they have reached the age of Chinuch. [Shraga Hameir ibid] Vetzaruch Iyun on why this was omitted from Dvar Halacha ibid. Perhaps, however, one can say that until a child reaches the age of puberty he is not considered to have reached the age of Chinuch for Yichud, as the age of Chinuch of a Mitzvah only begins when one reaches the age of understanding of that Mitzvah. [see 343:3] On the other hand, however, the age of Chinuch for negative commands, such as Yichud, should begin as soon as the child understands enough to be told that it is forbidden to do so [see 343:3]. Accordingly, the Mitzvah of Chinuch for Yichud by children with other children would begin at a very young age [from 3!], something which is unheard of. Thus, one must say that Yichud is not similar to other Mitzvos of Chinuch that begin as soon as the child understands the Mitzvah or understands the warning against doing so, as the Sages initially never enacted a Yichud prohibition towards children [which are mainly all below puberty] the same way they did not enact a Yichud prohibition when Baalah Bair, and hence this matter is not relevant to Chinuch at all. Vetzaruch Iyun.