Minors and Eruvei Techumin — Eruvin 82a
תלמוד בבלי מסכת עירובין דף פב/א
אמר רב אסי קטן בן שש יוצא בעירוב אמו
The Beis Yosef (Orach Chaim, end of §343) writes in the name of the Rashba and the Ran that one may deliberately feed a minor something that is prohibited by rabbinic decree — even if the child has reached the age of chinuch. However, the Rashba stipulates that his opinion should be taken as theoretical, not practical (להלכה ולא למעשה), and the Rambam rules explicitly that a child may not be fed a rabbinically prohibited substance. Moreover, adds the Rambam, it is forbidden to allow the child to habitually violate rabbinic prohibitions on Shabbos and Yom Tov ((איסור שבות.
The Beis HaLevi (3:55) notes that our Gemara seems to contradict the position taken by the Rashba and the Ran: Here we see that a child — even one that is not yet at the age of chinuch — must be a part of an eruv techumin in order that it be permitted to bring him beyond the techum. But according the Rashba and the Ran, since techum is a rabbinic prohibition, one should be permitted to remove the child beyond the techum even without his being a part of an eruv techumin, and even deliberately!?
The Beis HaLevi resolves the apparent contradiction of the basis of Teshuvos HaRashba §92, in which the Rashba writes explicitly that the dispensation to deliberately feed a minor something that is prohibited by rabbinic decree is only effective where the food in question (or the rabbinically prohibited activity in question) meets a need of the child himself. But when the child has no need for the food or activity, it remains forbidden to engage him in that food or activity. Evidently our Gemara concerns a case in which the extension of the techum is a need of the adult, not of the child. That is why the child must be a participant in the eruv techumin.