Kayin and Hevel were born before Adam and Chava sinned and ate from the Tree of Knowledge. (See Rashi to 4:1.)
Did Kayin and Hevel ever receive the understanding that their parents got from the fruit?
If not, how did they know to being sacrifices? And what about when Kayin admits that he sinned killing Hevel?
Derech Hashem 1:3:8 writes
אמנם על ידי חטאו – נוספו ונתרבו חסרונות בעצמו של אדם ובבריאה כלה
Through [Adam's] sin, he added and increased deficiency in himself and in the entirety of creation
Adam's sin did not just affect himself - it had cosmic impact on the entire nature of creation. If his action impacted the entire creation, it seems it was not necessary to be the one who did the "eating" in order to be subject to the effects.
On a separate note, you are assuming that the דעת that came from eating from the Tree of Knowledge was general דעת, without which they could never have figured out to bring a sacrifice. However, it wasn't the Tree of Knowledge - it was the Tree of Knowledge of Good and Evil. I discussed here what the Rambam understands that to mean, but briefly it had to do with the blurring of moral awareness. Nefesh HaChaim (1:6) understands דעת to mean "connection," and eating from the Tree caused them to internalize their struggle with Evil. Either way, their ability to bring Korbanos, or to choose to serve Hashem, were not the results of this event.
Rash"i that you cited talks only about Cain. Hevel was born another time, that possibly was after the sin (see Tosafot Sanhedrin 38:2)
When Chava gave the fruit to Adam she also gave it to the animals.
Bereshit 3:6.
ותיתן גם לאישה
רש"י: גם - לרבות בהמה וחיה
Translation: and gave also to her husband
Rash"i: "also" - to include animals
I'd suppose that Cain was not worse then animals, so he also eat from the fruit.
