Did mama notice anything different about Stacy and Cassie?
There's not really a moment, per se, that Mary Logan (Mama), "notices" something different about her two children, Cassie (narrator-9 years old) and Stacey (oldest child-12). It's a realization that her children are growing up as the story progresses. This is, after all, a "coming of age" story. And throughout, these two characters demonstrate their coming of age.
We can sense Cassie's maturation through her words and thoughts (as narrator) as well as her actions. Stacey's coming of age emerges through Cassie's narration and his interaction with T.J. and Mr. Morrison (as two examples).
Mama realizes these changes too. She comes to an appreciation for Stacey's growth as he demonstrates that he is capable of setting an example for his younger siblings, his dealings with T.J., and in passing thorugh the various "rites of passage" he encounters (e.g., the "coat incident" with Mr. Morrison, going against his father's wishes and going to the Wallace store to track down T.J. after T.J. got him in trouble with his mom for "cheating," and in "turning a deaf ear" to Cassie's inferred revenge plan for Lillian Jean).
The nature of the Mrs. Logan character (hard-working, caring and loving mother, counselor, teacher) works well with the author's way in getting the reader to infer the changes that occur not only in the characters of Cassie and Stacey, but also the way in which the entire family comes of age.