The Rema Yoreh Deah 81:6 writes:
לא תאכל המינקת אפי' ישראלית דברים האסורים וכן התינוק בעצמו כי כל זה מזיק לו בזקנותו
The wetnurse, even a Jewish one, shouldn't eat forbidden foods. Similarly the child themself, since this is damaging to them in their adult years.
My question is on the latter part of this statement. The Shach (§ 26) explains that although if a child eats something that is not permissible, there's no obligation to stop them, nevertheless, the father should. Even if it's only Rabbinically forbidden, the father should stop the child, because the child eating it will cause timtum halev (some sort of psychological or spiritual damage).
There seems to be the concept of timtum halev with regards to foods the Torah forbids (see for example Rabbeinu Bachaye). But how do we know that this applies to Rabbinically forbidden foods as well?
It's not clear to me if the Shach understands this to be the Rema's intent. I couldn't find this latter statement in the Darkei Moshe, so I don't know his source. What is it?