“Not necessarily. Many people only hold CY as a chumra, and are not always so makpid on it. For instance, some eat powdered milk, some eat CS when it’s difficult to get CY, and most eat things made with kailim used for CS. “
From what I have read from posts in the CR on this subject, those who are makpid on C”Y posted their mekoros, and it sounded very much that they were equating C”St with something that is not considered kosher. So if I held strictly by this (and they did not refer to it as a chumrah, but as halacha), I would not allow my kids to eat anything at all that was not C”Y, except in an emergency. (I love M&Ms , but there has never been an M&M emergency as far as I know).
I’m not saying they are correct, or that there are no exceptions, because as I said, I don’t specifically drink C”Y, so it is not shayach to me (except during aseres yemai teshuvah when I DO drink only C”Y) For chinuch alone, I would not vaccilate.