I'm not that good about grading scenes in movies, but at least i'll try. And I'll just use your grading style.

Here goes!
Atmosphere: Rain and lightning just blends in the scene, it's a great choice for the atmosphere of that scene. Also with the dim light in the place makes it more emotional to viewers. 5/5
Length: 2 minutes and 33 seconds exactly. It's kinda short for a sad and tragic scene, most tragic scenes should be around 4 minutes and up (i think) If the scene was longer, Littlefoot would have a chance to talk to his mother longer, before passing away. 2.5/5
Acting: It's perfect, they did it really great that you can feel the emotion of Littlefoot when he was talking to his mother. 5/5
Music: "Whispering Winds" (The name of the music) oh come on! I love the music in that scene! It perfectly blended with the scene. It also made the emotions stronger. Yeah, that explains most people cry on this scene. I almost did cry on that scene. 5/5 (I actually wanted to make it 6/5

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Dialogue: His mother was giving him the inspiration to find the Great Valley, and also was giving him inspiration that she will always be with him, even though if he can't see her anymore. I'd say the dialogue is perfect. However, for the last line which is "Let you heart Guide you. It whispers...so listen closely" That really shouldn't be the last words. It should be something that is really meaningful to Littlefoot. And like I said in the "Length" part, it should be longer, that means if it was longer then there will be a much better line as for her last words. 4.5/5