The very sensible Catherine has made it clear she wants her children to have a normal school upbringing with other children of the same age, with no special treatment. So they are known as George and Charlotte Cambridge. William is of that mindset anyway; he was plain William Wales in the armed forces.
Like so many things with the British Royal family, its modus operandi is constantly being updated but few people notice the joins. The Queen was educated at home; but her children went to school and Prince Edward was the first to take a normal university degree (He read History at Cambridge). Catherine will be the first Queen to have a degree herself; after all she and William first met at St Andrew’s university in Scotland.
In any case, Princess Charlotte is unlikely to need royal titles as a prop. Even as small child she gives the impression of eventually growing into an exceptionally good-looking woman. Then again, if you look at her mother and two grandmothers, that would not be all that surprising.