"We have cookies."
Translation:My mamy ciasteczka.
"ciasteczka" is a diminutive form of "ciastka", so it's kinda like "little cookies". I would personally definitely teach "ciastka", but the people creating the course decided on "ciasteczka".
"ciastka" and other appropriate forms should always be accepted, if we forgot to add them somewhere, please comment/report there.
There is no such thing as
ciasteczkę, you've just made that up. And the nominative plural is defninitely ciasteczka.