Louis César would grow up with his older brother Louis-Auguste in the care of Madame Scarron in a house on the rue de Vaugirad in Paris. This house had been purchased by the king specifically for his illegitimate children. The next year, while his father was on a military tour at Tournai, he was joined by a sister, Louise Françoise de Bourbon, born in June, Louis César and his parents, as well as the Queen Marie Thérèse d'Autriche were present as was Madame Scarron.
Scarron was greatly attached to Louis Auguste, not so much Louise Françoise or Louis César. On 19 December 1673, Louis XIV officially recognised his children with Montespan with Letters Patent from the Parlement de Paris. This made all these children Enfant's Légitimé's de France, Legitimate Children of France.
With this legitimisation, Louis César received the title Count of Vexin, an ancient title dating from the 10th century. His brother became the duc du Maine and his sister Mademoiselle de Nantes.
From early in his life, Vexin had had a crooked spine since birth. This caused him to limp and was not helped by one of his shoulders being higher than the other. His father, who adored him, decided the child was to be destined for the church. His father bestowed the title of Abbot of Saint Denis on his son. The Royal Cathedral of Saint Denis, Cathédrale royale de Saint-Denis, was the traditional burial place of the kings of France since the 7th century and was one of the wealthiest churches in France.
Despite being the Abbot of Saint Denis, he was too young to carry out any duties and thus remained at the court of his father whilst under the care of Scarron. In 1674, another sister joined the family: Louise Marie Anne was born in November 1674 and was created Mademoiselle de Tours after her legitimisation in 1676.
The doctors at court tried to help with Louis-César's condition but failed using the awful treatments. After such treatments, Louis César's general condition worsened after 1675. He was watched over by his mother and his maternal aunt, the marquise de Thianges. He would never be a strong child.
In 1678, it was thought that he may die; again he was "treated" but did not do anything other than keep him alive. His condition was so bad that he was bedridden for days at a time.
Around the time of Françoise Marie, his mother was supposedly involved in the Affaire des Poisons which made her lose favour with the king, who sought comfort with another mistress, Angélique de Fontanges. After that time, he spent more time with his mother at her private residence, the Château de Clagny. His mother stayed with him much of the time.