Indeed. I doubt that we'll see the twins grow a little older. I think we'll see the birth, Luke taken off to Tatooine and Palpy in full control of the galaxy and that's it.
As for Leia's age, if we can apply what we know from real life, we know that people don't remember anything (or hardly anything) from before the age of four. So my guess is that Leia was somewhere between 3 and 5 when her mother died.