At the moment, it's all grandchildren of a monarch in the male line
Which, when William becomes king means that both Goerge's and Louis' DC would be HRH Prince/ss, but not Charlotte's. Which is a nonsense, as her DC would be higher up in the succession.
So it will need attention idc (but could be safely left for a decade, or even two!)
Two obvious options:
a) all grandchildren of a monarch have royal styles
b) only the children of the eldest child have them (ie sticking more closely to the line of succession - meaning in this generation only George's offspring)
Might need a further tweak if George has no DC, and therefore offspring of the next sibling are the de facto line, but that's a matter so far in the future that it doesn't need considering now.
I think the King would go for option b. But might be waiting to see what the Sussex view is. Queen Margarethe has just removed royal styles from 4 of her grandchildren, and whereas I don't see KC removing them from all the little Waleses, I do think they might cut off those outside the direct line from passing them to their DC (even though under current rules they would get then when Willian succeeds)
Best done before any of these hypothetical DC are born though, so no later misunderstandings.
I think best solution for the Sussexes is to do as the Wessexes did