Would you be offended???

I would have smiled and said no thanks, she doesn't eat that brand of cereal.

Edited to say: And quite honestly yes I would be offended because what would make her think you would want or need an open box of generic cereal for your household? I know many say they wouldn't take offense, but if she is two faced like you said, she obviously has a foul history so she would definitely have offended me.

Thank you. I am glad I am not standing on this ledge alone. I was a little offended by this. Like I can't afford to buy my child cereal. Like I said there is another mother in our dept she didn't offer it her. Why me?

I would have either said

a) thank you ...but no thank you (politelty though)
or
b) accepted them, said thank you...and threw them away when she wasn't looking.

Sometimes you have to kill people with kindness, especially in the work force cause SOME of them mofo's think the word "supervisor" gives them a green card to treat people under them like ****

Oh so you met my boss then....LOL that is her to a tee.

Since I know how you feel about your boss.  IMO, I think you are reading too much into it.  It's just cereal; and yes your child eats cereal.  I would have said thank you and thrown it in the garbage when she wasn't looking.  But what she said I would have been offended by....'Here she can have this then I don't want it'.  Like she's doing you a favor.  Thanks but no thanks!!
  

I find your response funny. I would have thought you would have said the opposite.

I don't get it...opposite how?  Don't know why people would think that we want their opened box of cereal.  You wouldn't buy an opened box at the store, and they won't even give you an opened box at the food bank..so I don't know.  Maybe she was either being funny or thought she was doing something nice.

Sorry I misread I thought you were saying I should take the open box of cereal.

I would have taken it said thank you and then ditched it later .........open box

My sentiments exactly.