When a student from Oholei Torah of Kfar Chabad came into yechidus with his father, the Rebbe began talking to the boy in Yiddish. When the boy responded in Hebrew, the Rebbe expressed surprised that he didn’t speak Yiddish. After that, the Cheder changed their language to Yiddish.
Oholei Torah Cheder of Kfar Chabad was established in 5733 by a group of Chassidim, led by Reb Mendel Futertfas, with the goal of having a curriculum free of secular studies.
In 5736, the cheder menahel, Reb Bentzion Vishedsky, was in yechidus with his seven-year-old son. Addressing the boy in Yiddish, the Rebbe asked him questions on what he was learning, but the boy struggled to understand and needed his father to translate for him. He gave his answers in Ivrit.
But the Rebbe continued talking to him in Yiddish.
After discussing his learning, the Rebbe asked him what color his jacket was. Now it was already clear that the Rebbe was testing his Yiddish. The Rebbe continued asking the color of his sweater, and the boy responded in Ivrit.
“How does he know Loshon Kodesh so well?” The Rebbe asked Reb Bentzion. “Doesn’t the Cheder learn in Yiddish?”
From then on, Oholei Torah of Kfar Chabad changed its spoken language to Yiddish, and thus became known as “Cheder Yiddish.”
(Choma Zu Torah Teshura 5757)
From The Weekly Farbrengen by Merkaz Anash
here is a collection of guidelines from the Rebbe regarding this subject
click here:
https://drive.google.com/file/d/1LI-VOyZ0AAUN5JLLq38_UxiBU1dFmG1v/view?usp=drive_link
here’s the recording of the yechidus in 5732 where the Rebbe tells Frank Lautenberg that he is not in favor for the Yeshivos insist on teaching in Yiddish to English speaking students
https://www.chabad.org/therebbe/article_cdo/aid/551695/jewish/Yechidus-Frank-Lautenberg.htm
Or click on this link: tinyurl.com/Yiddish-or-English
The Rebbe’s responses about teaching in English was addressing older students who are not inclined to learn a new language. In that case, the Rebbe instructs focusing on the content instead of losing the student. That isn’t relevant to young children who can absorb a language very quickly.
Please read the article.
You will see it’s talking about elementary kids too.
On the other hand, if they made total immersion in preschool, then there would be no problem teaching in Yiddish in elementary