Several questions arise concerning this pasuk:

1) Why does it repeat itself – “Say… and say to them”?

2) If the laws presented here are addressed to “the kohanim, the sons of Aharon,” why does the pasuk continue in the singular form – “one shall not become defiled to a body… “?

3) Why must the kohanim be identified as “the sons of Aharon”; do we not know this already?

Rashi, commenting on the words, “Say… and you shall say,” writes that Moshe here is bidden to charge the adults with the responsibility of ensuring compliance with these laws on the part of the youngsters. Meaning, the adult kohanim must instruct the children with regard to the laws of tum´a and tahara (ritual purity and impurity). Undoubtedly, this is no easy task; these laws are particularly complex and intricate, especially for young children. The teacher must therefore exercise utmost patience when presenting the material. If he is impatient or irritable, they will obviously not learn much from him. If in general Chazal tell us in Pirkei Avot, “lo hakapdan melamed” – the short-tempered cannot teach, certainly with regard to these laws a teacher must be patient and composed for his instruction to be productive. This is indeed the quality of Aharon Hakohen, the “lover of peace and pursuer of peace.” The short-tempered individual is not a man of peace; more often than not, he is entangled in controversy and dispute, as he cannot tolerate the words or actions of other people, prompting him to battle against everyone around him.

It may therefore be suggested that the clause, “the kohanim, the sons of Aharon” indicates that not every kohen is qualified to teach the children. The job is suited only for those who posses the quality of Aharon – “ohev shalom,” a love of peace. To specifically these kohanim the pasuk refers when it says, “the sons of Aharon.” These adult kohanim are worthy of this distinction, for only with such qualities can one teach youngsters. We can now understand as well why the pasuk first addresses the “kohanim” and then shifts to the singular form. The “kohanim,” the teachers, who instruct the children with regard to the intricate laws of tum´a and tahara, must study with each child individually; the classroom setting is not enough. The pasuk therefore utilizes the singular form – “he shall not become tamei” – emphasizing the need for individual, personal instruction.

Taken from the work, “Hadrat Yirmiya”