Before the Queen died Archie was entitled to be either Earl of Dumbarton (his father's secondary title) or simply Lord Archie - as the son of a Duke. Lili is entitled to Lady Lili as the daughter of a Duke.
Opinions seem divided at the moment whether as the grandchildren of the monarch, but born before he became King that they are automatically entitled to be Prince/Princess or whether Charles has to formally confirm it. Either way their parents released a statement after Archie's birth saying they didn't want him to use a title so I assume that stands for both kids now.
I think Charles should issue a new Letters Patent which restricts HRH/Prince/Princess titles to the children of the direct heir line. So only George's children etc etc.
Then children of Charlotte and Louis etc know where they stand from the start, they can still be styled as Lord/Lady etc and if needed step up into more formal roles but not expect to.
I'm not sure if it should be retroactive though - if you do that you'd have to apply it across the board and I don't think anyone wants to be the one to take Princess Anne's title away!