Ya Never Know:
I don't think you have anything to worry about. DD and I shared a portobello mushroom sandwich the other day and she loved it! She's been eating mushrooms since she started eating table foods. The only thing my doctor says they shouldn't have before 1 year is honey.
My aunt gave LO mushroom empanadas at Christmas. I honestly didn't think a thing about it. She loved them. Like above post, the only big no no is honey.