@kahorifutunaka in September of 1865, while in Mexico, Maximilian reached to an agreement with the descendants of Agustín Iturbide, the first emperor of Mexico, who were living in exile in United States. He elevated to the rank of Princes of Iturbide the two grandsons of the emperor, Agustín, aged two years old, and his cousin Salvador, aged sixteen, and took them into his custody. Their aunt Josefina, the only surviving daughter of Iturbide, was also made a princess, and co-tutor of the children.
Salvador was sent to Europe to study, but the little Agustín remained in Mexico and was brought up by his aunt in close proximity to Max and Charlote, who looked over the boy's education.
The little Prince Agustín de Iturbide, around the age of 3. Via Wikimedia Commns.
It was believed that Max's plan was to adopt the boy and make him his successor in case he didn't have children, although this never came to be. Charlotte, on her part, wasn't thrilled by the business. She wrote to her grandmother Queen Marie Amélie:
Several members of the family of the Emperor Iturbide have been raised to the rank of princes. All sorts of predictions for the succession have been seen in this; it is only an act of justice on the part of an Emperor seizing the throne to take under his protection the descendants of a dethroned Emperor who was not of royal blood. We have at the Palace, while waiting for a home to be prepared for them, Princess Josephine Iturbide, raised in the United States, and little Augustin, a child of two years, fresh and rosy, not very well brought up until now. The other, young Salvador, aged fifteen, his cousin, is raised in Paris, at the college of Sainte-Barbe, from where he will probably enter Saint-Cyr and become, I hope, a French officer. The rest of the family is returning to Europe. The sons of Emperor Iturbide had contracted in the United States habits of gambling and drinking which made their elevation to the same dignity as their nephews impossible.
This is the heart of the matter in the event that the newspapers considered it differently.
CHARLOTTE
It is obvious from her tone that the idea of the boys being put into the line of succession upset her, unsurprisingly since that would mean that Max had given up on having biological children with her.
There was however another person who was even less thrilled by the business than Charlotte: Agustín's mother, an American woman named Alice Green. She only agreed to Max's deal because of the pressure of her in-laws (the rest of the Iturbides got nice pensions out of the agreement), and even asked if she could remain with her son at least until he was five. Not only Max didn't agree, he actually was very clear in that all the family should leave for Europe, Alice included. He regarded her as "a half crazed American" and thought it would be perjudicial for the child to be raised by her.
As time passed and Maximilian and Charlotte's position became dim, Alice, who was living in Paris, grew more and more worried for Agustín, to the point that she asked Max to please give her back her son. He agreed to her visiting the boy in Mexico, but refused to return him. Alice however had already written to the government of the United States, demanding for their intervention to guarantee the return of her child. Understandably, the mother was desperate, she even broke the agreement and went to Mexico, demanding to see her son, but she was forced to leave without seeing Agustín.
This was awful press for the Mexican empire and Max, as he was now being seen as kidnapping little Agustín (which technically he was lol). He eventually relented and on October of 1866 he wrote to the mother, telling her he would return the boy. And thus ended the short life of Agustín de Iturbide y Green as a Mexican Prince.
Corti, Egon Caesar Conte (1993). Maximiliano y Carlota (translation by Vicente Caridad)
de Reinach Foussemagne, Hélène (1925). Charlotte de Belgique, Impératrice du Mexique