Giving a child an item with which he may commit a prohibition:[1]
Non-Kosher food: It is forbidden to give a child [of any age] non‑kosher food if there is a chance that he may come to eat.[2] [This applies even to food forbidden merely due to Rabbinic decree.[3]] Moreover, even food that is prohibited only during a specific time period[4] may not be given to a child [when it is forbidden for him to consume it at that time] if there is a chance that he may come to eat it.[5]
Foods prohibited due to repulsiveness:[6] Even foods that are forbidden solely because of their repulsive nature, and thus fall under the prohibition of bal teshaktzu, may not be given to a child if there is concern that the child may come to eat them.
[1] Admur 343:9 “It is forbidden to give a child a live non‑kosher locust to play with, lest it die and the child eat it, in which case one would be actively feeding him a prohibited item, since he himself gave him the prohibition. All the more so is it forbidden to give him any prohibited item that is fit to be eaten as is. Even something that is prohibited only because of bal teshaktzu (repulsiveness) may not be given to him if there is concern that he might eat it. However, it is permitted to give him a non‑kosher bird to play with, since even if it dies it is not fit for consumption.” Admur 343:10 “(All of the above applies to prohibitions related to food, even when the prohibition is time‑dependent.) However, on Shabbat it is permitted to give a child objects with which he can perform a prohibited labor for his own benefit, even if it is known that he will do so. For example, one may give a child a cake with letters written on it, which is forbidden to eat on Shabbat, as explained in siman ש״מ. Even though the child will certainly eat it, since his intent is for his own enjoyment, there is no obligation to prevent him, as explained above—provided that an adult does not place the food directly into the child’s mouth.” Admur 616:4 “All of this applies to a minor who has not yet reached the age of education for the mitzvah of affliction. However, if he has reached the age of education, it is permitted to feed him, give him drink, anoint him, and wash him directly, in the manner explained above. If the minor performs any of these acts on his own, there is no need to protest, except for his father, who is required to protest and to educate his son in the mitzvot. His mother, however, is not obligated in this. Nevertheless, it is forbidden for her to place food before him, since doing so is considered as if she were feeding him directly.”; Kaf Hachaim 343:13; Piskeiy Teshuvos 343:4
[2] Admur 343:9 “All the more so is it forbidden to give him any prohibited item that is fit to be eaten as is… if there is concern that he might eat it.” and 10 in parentheses “All of the above applies to prohibitions related to food”; M”A 343:3; Hagahos Ashri Shabbos end of chapter 9; Shabbos 90b; Kaf Hachaim 343:13; Toras Hakatan 19:1-29
The reason: As this is considered as if he is actively feeding him a prohibited item. [Admur ibid; M”A ibid; rashi ibid]
[3] As rules Admur next regarding Bal Tishaktzu
[4] Such as Matzah on Erev Pesach for a child who has reached the age of understanding, or a child on Yom Kippur which has reached the age of Chinuch for fasts.
[5] Admur 343:10 in parentheses; 616:4
[6] Admur 343:9; M”A 343:3; Implication of Shabbos ibid; Tosefes Shabbos 343:3; Kaf Hachaim 343:13; Ketzos Hashulchan 147 footnote 10; See Admur Basra 3:11