It is a mitzva to train children to avoid prohibited activities from the time they begin to understand that certain things are permitted and certain things are prohibited. In other words, it is not enough that a child understands that he must stop what he is doing when he is told “no”; rather, he must understand that what he is doing is never allowed. Most children begin to understand this at approximately age three. From then on, if one sees his child engaged in a prohibited activity, such as eating non-kosher food or turning on a light on Shabbat, one must stop him from doing so (MB 343:3). Once the child reaches the age of ĥinukh – when we start teaching him Torah at about age six or seven – one should begin to explain more about the principles behind the prohibitions, so that he will know how to observe them properly.
There is no mitzva to teach one’s child who is younger than three to avoid prohibitions. Therefore, if such a child finds prohibited food and wants to eat it, or if he wants to turn a light on or off on Shabbat, one does not need to stop him from doing so. Similarly, if one’s small child is a kohen, and he goes to a place of ritual impurity (such as a cemetery), one does not need to stop him, since he does not understand the prohibition.
All of this refers to situations in which the small child is acting autonomously. In contrast, if an adult causes a child, even a day-old baby, to do something prohibited, the adult violates a Torah prohibition, for the Torah commands us not to cause a child to violate prohibitions. Thus one may not feed a child blood or insects or bring a young kohen into contact with ritual impurity (Yevamot 114a; MB 343:4). An adult may not even feed a child rabbinically prohibited food (SA 343:1).
However, it is not prohibited to give a small child an item that he might use in an impermissible fashion. For example, one may give paper to a small child on Shabbat even though it is likely that he will tear it and destroy letters that are written upon it, as giving him paper is not considered the same as instructing him to tear paper. However, one who puts non-kosher food into a small child’s hand, though, is considered feeding him, as this is the normal way to feed a child (MA; MB 340:14).
On Shabbat, there is an additional prohibition, for we are commanded not to have children undertake melakha on our behalf: “But the seventh day is a Shabbat of the Lord your God; you shall not do any melakha – you, your son or daughter” (Shemot 20:10). This means that if a child turns on a light because he thinks this is what his parents want, and his parents know and do not object, then in addition to neglecting the rabbinic mitzva of ĥinukh, they are also transgressing a Torah prohibition by having their child do melakha for them. If a child turns on the light for another Jew (other than a parent), who sees him doing so and does not object, that Jew transgresses rabbinically (SHT 334:54).
Even though we have seen that one may not feed a minor forbidden food, if a child is hungry or thirsty before kiddush or havdala or during a fast, then even after he has reached the age of ĥinukh, the adult may give him food and drink. It is only forbidden to feed a child food that is intrinsically non-kosher. If the food is kosher but rendered unfit by the time, a child who is hungry or thirsty may be fed (MB 269:1; above 6:9).