Well she likely would be in her early teens by now I would think, given that she was young enough to be having a children's birthday party when they bombed it, but obviously old enough for Douglas to have seperated from her mother and re-married again in the interim.
I'd have guessed she was between 6 and 9 during the carpet-bombing carp incident and, as time passes in Fitton the same way it passes in real life, that would put her in or near her tweens by now.

But in my head, Douglas's '8 years sober' coincide with his daughter's birth so that does give her a fairly specific age as far as I'm concerned.