Twitter almost universally supported the ice cream idea, with Mason eventually posting a photo of his daughter with her treat for her clever cheekiness.
Users applauded her "anti-authoritarian" thinking, saying she should get an allowance raise, a year's supply of ice cream or be put in charge of the country.
Some suggested it was a sign she would grow up to be an independent thinker and was destined for law school - although a few contended that it could equally indicate she would end up "in the penal system."
Mr Cross later added: "I should clarify that she thinks her teacher is awesome - it's just this aspect of the educational justice system she has an issue with."
Even if Ava was guilty of exaggeration, one Twitter user pointed out that she had at least educated people worldwide about the Geneva Conventions, which are at the centre of international humanitarian law and apply during times of conflict.
The conventions, which have been ratified by 196 countries, prohibit torture, attacking civilians, treating detainees humanely and minimising the impact of warfare on children.
Her note has clearly given her father's career a boost, with an Argyle bookshop tweeting a photo of his book with a new sign saying the novel was by "Mason Cross (Ava's dad).
