Musar do Kapłańska 22:21

אותו ואת בנו לא תשחטו ביום אחד . The Zohar (Sullam edition page 24) poses the question that if this legislation of not slaughtering the young animal on the same day as the mother animal (22,28) is to save the mother animal the pain of watching its young killed, this could be avoided simply by keeping them apart. The true reason, however, is connected to the Jewish peoples's sense of empathy. To the extent that a person displays consideration for the feeling of others he in turn may find that such considerations of his own feelings will be a factor when he will be judged. The reverse is also true.

“There must be no blemish in it” (Leviticus 22:21). We were warned with this not to introduce a blemish into a firstborn - even in our times.