So, were the cookies?

I sure thought they would turn out to be. But we never found out so they're just a red herring - or very poor directing....

I thought they'd be poisoned too. She probably won't eat them now anyway...

I thought so too, would've made the neighbors' plans go a whole lot smoother. When Grace was handing those cookies over I bet she wasn't bargaining on having to shoot her way to the family or losing one of her party's members in the process. And just a thought, given the community that they were in, I bet there's a awful lot of competition between the mothers... Mary probably tossed the cookies in the trash asap, not wanting to risk the kids or her husband finding out that Grace might cook better than her.

It was a day like any other except it was 14 hours long and everybody was wearing pastels

That would have been wrong to give poison cookies prior to the purge. If she ate them right away she would have died before it was legal to murder.

No, it was just a way to introduce Grace so we could immediately get (at least I did) that Grace planned to "purge" the family.