Let me see if I can articulate how I feel about it...obviously sad things happen to kids and kids understand grief and loss. But I think for a lot of us, watching it when we're so young, some of the other topics covered in the film (like aging, passing of time/"circle of life" etc) are so abstract that while we can appreciate how sad it is that Littlefoot's mom dies, we can be detached from the other heavy aspects of it.
But watching it as an adult, you're very aware of your mortality, likely more acquainted with loss and grief, and better understand how fleeting life is and I'M ADDING A CUTE GIF HERE TO DISTRACT MYSELF FROM THE FEELS