Yevamot 114B

משום דאיסורן במשהו, אבל דם ⁠— דעד דאיכא רביעית, אימא לא. ואי אשמעינן דם ⁠— משום דאיכא כרת, אבל שרצים, אימא לא.

we would have said that the halakha is stringent in this case because they are prohibited by any amount, i.e., a creeping animal or insect is prohibited on pain of lashes even if it is very small, provided that it is a whole creature (Tosafot). However, with regard to eating blood, which is not a Torah prohibition unless there is at least the volume of 1/4th-log, one might say that there is no prohibition against directly feeding minors, as this prohibition is more lenient. And had the tanna taught us only the case of blood, we would have said that they were stringent with regard to blood because there is the punishment of karet for eating blood. However, for creeping animals, whose consumption does not entail karet, one might say that this halakha does not apply.

ואי אשמעינן הני תרתי ⁠— משום דאיסורן שוה בכל, אבל טומאה, אימא לא. ואי אשמעינן טומאה ⁠— כהנים שאני, משום דריבה בהן מצות יתרות, אבל הני, אימא לא, צריכא.

And had the tanna taught us only these two cases, we would have said it is because their prohibitions apply equally to all Jews. However, in the case of ritual impurity, which is a mitzva for priests alone, one might say that there is no prohibition against directly rendering minors ritually impure. And had the tanna taught us only the case of ritual impurity, we would have said that the case of priests is different and more stringent, because the Torah includes extra mitzvot for them. However, with regard to these prohibitions of creeping animals and blood, one might say that there is no prohibition against directly feeding minors. Therefore, it was necessary to state all these cases.