This is stunning. My first moment of perspective was from her firstborn. I saw him as delightedly participating in his mom's and siblings' Sunday jaunt, only unseen by them.
But not unfelt by Mom. Because she lives with his memory every single day. She never forgets about him, and his missing presence is acutely in her mind always. At first glance, her expression seems to show a look of peaceful pleasance. But when I look closer, I see a slight frown, so I assume she's thinking of him in that very moment, missing him, feeling the group is incomplete.
The rope between them just represents the unbreakable bond that keeps a mother and her deceased child together forever. It's a good thing that allows her to safely go on with her life. Contrarily, she could not do that very well if she thought she 'should' succeed in forgetting about him. A bereaved mother's biggest fear is that she will forget anything about him, most of all, his face and his scent.
My only 'complaint' is that the coffin seriously adds to the acute pain of the scene. This is a harsh reality, true. It makes for a very gritty reality. I'm not a bereaved parent, and it makes me a little uncomfortable. That probably says more about me than anyone else, and it's the artist's vision anyway, of course. I do wonder if the same emotion could be expressed another a way without a coffin, for an alternative version.