As someone who has a daughter in college, I thought it was about learning how to let go. The mom wants to keep her son with her and enjoys the relationship she has with him when he's small like your kids. As he grows, he naturally distances himself from his mom (I loved the teen Bao-boy, BTW) and hangs out with friends. Let me tell you, it's no fun when family takes the back seat to your children's friends and of course she is sad. When he finally becomes engaged she is afraid that she will lose him forever. So, she eats him to keep him with her always.
I found this that explains what the director was going for (there are some naughty words in the article):
https://www.thrillist.com/entertainment/nation/bao-pixar-short-film-before-incredibles-2-explained
FWIW, I wrote my analysis before I read what the director was thinking
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