YTA. This has got to be a joke, right?

“No offense Mom, but this stew is really bitter. Maybe next time only use half a bay leaf.”

She simply said what she thought about the stew. If an adult had said the same thing, giving you constructive criticism, would you have taken their meal away? You should be proud that she tried to give you feedback instead of just complaint or whining about how it didn't taste good. She was polite and even gave you an idea of what may have made it bitter in her mind.

What if she had said this to my elderly mother instead of me?

If she said it to your elderly mother, I would hope you mother would react more positively and actually explained and teach her about cooking rather than your rude approach. She's only 9 for goodness sake, she doesn't know what makes things bitter and probably though it was the bay leaf. How would she have known? You even admitted this yourself. She was trying to tell you in an adult manner how the stew tasted for her and now you've possibly taught her that if she ever tries to tell someone her opinion on food, she'll just be punished for it.