Royal rules: Princes George, Louis and Princess Charlotte not allowed to have dinner with William, Kate

livemint.com:

A distinct set of guidelines govern the behaviour of the young members of the Royal Family, setting them apart not just in public appearances but also within the confines of their residences. Embracing Royal etiquette from an early age, Prince George, Princess Charlotte and Prince Louis are not exempt from the rules that define their upbringing. Among these traditions is the curious practice of segregating the young Royals from their parents during formal gatherings, even during festive occasions like Christmas.

So, this applies to the children of Prince William and Kate Middleton as well. According to Harper's Bazaar, the young heirs are placed at a designated children's table rather than alongside their parents at official dinners. This custom is said to persist until they learn “the art of polite conversation", an art deemed requisite for mingling with adults.

This implies that even on the most celebratory of occasions, such as Christmas feasts, the young ones are ushered away to partake in the company of their peers. However, this practice extends beyond formal events. Reports from within the palace revealed to the publication that past Royal children, including Princes William and Harry, underwent a similar upbringing.

A former Royal chef, Darren McGrady, shared insights into the Royal dining habits. McGrady highlighted that the young Royals were nurtured within the palace's nursery, an educational haven that aimed not just to cultivate their minds but also to refine their tastes. McGrady disclosed that the palace's nannies meticulously crafted menus, striving for nutritional balance while introducing the youngsters to a variety of culinary delights.