I think it is quite possible - Haitian Kriole is derived from French, and 
French is still widely spoken.  Tok Pisin is derived from English and a 
number of Papua New Guinean languages, and both English and the Papua New 
Guinean languages are still spoken.

It's a matter of social context.  What are the social contexts of this parent 
and these daughter languages?  In one of my conworlds, I have a conlang 
called E'rava and a derived trade language called Tana'la - nearly everybody 
understand Tana'la; E'rava is still spoken as the common language of the area 
and triple city where it developed.  Though it is now developing in 
Tana'la-tinged ways that irritate the hell out of the aristocrats who view 
themselves as the defenders of the language-faith.  ( " ' " indicates the 
major stress; E'rava has primary syllable stress; Tana'la has penultimate 
syllable stress; consequently words tend to "warp", and consonants tend to 
get either voiced, or devoiced, depending on just where they are in proximity 
to the stressed syllable.)

I think it's possible - dig into English dialects - wish I knew just what 
books existed so I could recommend them to you - but English dialects - and 
probably other well-studied European, Middle Eastern, Indian and Asian 
languages - will be of most help in this.

Wesley Parish

On Mon, 16 Nov 2009, John Lategan wrote:
> Hello everyone,
> I'd like to ask a few questions regarding language evoulution within a
> conculture.
> The conworld is similar to earth and, for the moment, time is the same as
> earth-time. The first trace of language appeared about 5 000 years before
> the 'present'.
> So, now for my question:
> Can a parent language exist as an every-day language in one nation;  while
> its daughter languages are spoken in neighbouring nations?
> rephrased: Does a parent language have to die for its daughters to be used?
> A real-world example would be: Latin being spoken at the same time as
> French, Italian, Romanian &c.
> My second question is:
> If a language evolves, is it likely that it will loose case in favour of
> prepositions?
> I know Icelandic still has 'intense'-case, but solely because it is so
> isolated and wasn't influenced. If a culture is not isolated, and the
> language evolves, will it loose most of its cases (eg: the Causal,
> ablative, instumental and comitative cases)?
> please answer as if this language existed on earth, and survived/ evolved
> realistically.
> thank "youse"
> John Lategan

Clinersterton beademung, with all of love - RIP James Blish
George Kelischek - "To impress those high-tech computer types, 
tell them what an Ocarina really is: 
an animal-activated-solid-state-multi-frequency-sound-synthesizer." 
Mau e ki, he aha te mea nui?
You ask, what is the most important thing?
Maku e ki, he tangata, he tangata, he tangata.
I reply, it is people, it is people, it is people.