I believe Empress Eugenie would become Regent for Napoleon IV - she served in that role several times IOTL. French policy would turn a lot more conservative, I think. The fact that Italian nationalists were responsible for the attack would certainly sour the French on the notion of supporting Italian unification even without the Empress' sympathies with the Pope. For the same reason, she probably wouldn't like the idea of Prussia fighting Austria, so the 1866 war might get butterflied as well. Some big changes for sure.