Answer:
The child is in Piaget's pre-operational stage.
Step-by-step explanation:
This stage begins at the age of 2 and continues until the age of 7. As the name states, the child is not yet ready to form concrete operations and understand logic, which means that he/she still cannot fully manipulate information mentally.
A person who has passed this stage can comprehend that a cookie cut in half is still one cookie because that person understand reversibility, which is to go backwards on a central idea to its origin.
So, if the cookie is cut in half, the person can go back on the thought that the cookie is still one, not two. A child in the pre-operational stage does not understand this concept yet, therefore he/she thinks that he/she now has two cookies.