Do the laws of Yichud apply towards children? May a male and female child that is not siblings be alone together?
Children that 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 alone together. 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 it is proper due to Chinuch obligations to educate children that have reached the age of Chinuch to not be alone together on a steady basis.]
 Rashal Kedushin 22 based on Smag; Dvar Halacha 2/8; Shraga Hameir 8/126
 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 not become accustomed to be with each other”
The reason: As although the children themselves are not forbidden from the letter of the law to be together with each other, nevertheless this does not differ from 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, would begin as soon as the child understands enough to be told that it is forbidden to do so [see 343/3], accordingly then 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.