I would not be surprised to find that the title of prince(ss) of Belgium will be limited within a year or two and that the limitations might actually also count for the children of Philippe's siblings.
The children of Astrid have their father's title and the children of Laurent could use the titles of prince(ss) of Saxe-Coburg and duchess/duke of Saxony. As male line descendants of Leopold III they are entitled to do so. That would leave Josephine and Oscar with the surname of Belgium but no titles or royal styles.
If the royal title and style is limited to children of consented marriages than even Delphine will have to do with the of Belgium surname and she might try to claim the Coburg and Saxony titles but im not sure if Belgian noble laws limit titles to legitimate children and exclude extra-marital offspring. Otherwise that would be an option for her.
Delphine Boël has become HRH Princess Delphine of Belgium. The princely title and the style of Royal Highness also extends to her two children. See link (French): https://www.rtbf.be/info/belgique/detail_delphine-boel-est-officiellement-designee-princesse-de-belgique?id=10598252
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