Louis-Philippe's family received the title of Royal Highness before Louis-Philippe's reign.
They were Serene Highness until the 21 september 1824.
The 21 september 1824, King Louis XVIII accorded the title of Royal Highness to Louis-Philippe and to all his children, and to Louis-Philippe's sister Adélaïde.
For Marie-Amélie, she had already the title of Royal Highness, because she was daughter of the King of Two-Sicilies (and in the french court, she was more "high-ranking" than her husband. When they were in front of a door, she was going into the room before her husband, with the double-door completely open, and only after her, he came into, with the double-door just half-open!).
Then, when Louis-Philippe became King, his family had already the title of Royal Highness.
For the other part of the question, Louis-Philippe decided by an ordonnance, the 13 august 1830, that his children would continue to bear the name and the arms of Orléans, the sons with just their usual title (duke of Orléans, duke of Nemours, Prince of Joinville, duke of Aumale and duke of Montpensier), the daughters with the titles of "Princesses Louise, Marie and Clémentine of Orléans".
So they were styled Prince/ess of Orléans, and were never styled Prince/ess of France, Prince/ess of the French.
An exception : the heir, duke of Orléans, had also the title of "Royal Prince of the French" (Prince Royal des Français).