THey'll all do fine if you teach the girls to have respect for your girl.
She is old and they need to be careful. Many people in this world (including
myself) were raised with cats, fully clawed and got on just fine. It would
be nice for your cat to have her own "safe zone". That will keep her stress
level during the change down.

Karen

I only forsee a problem with your suggestion of segregating them if your
fiance has an issue with you diciplining her children.  That's an issue that
often arises in a stepfamily situation, and one that is talked about on the
front end a lot less than you would think.  Your fiance may find the idea of
you correcting her children's behavior intrusive. You both have to agree on
how the children will behave and the measures that you will take to ensure
your cat's safety as well as the childrens.  Now, as long as your fiance is
not an insane creature from Mars and her children devil spawns of satan
himself, you ought to be able to come to an agreement.  ;~) However, never
discount the possibility that a two and a four year old won't have at least
a few devil spawn tendancies. Most normal kids do. But, if those heads start
spinning around, run the other way!

Rob,
Congratulations in advance on your new family.  The situation you describe
sounds like everything should work out fine. As long as the cat has a place
or places that are considered hers and the girls are taught to respect the
cat's personal and architectural spaces, they should not have any problems.
If the cat is still agile enough, give her handy places that are too high
for the girls to reach.
While providing private places for the cat, encourage supervised interaction
with the cat and the girls.  Explain to the girls that they look very big to
the cat and the cat may have to watch them for a while before she will feel
comfortable about letting them touch her.  Let them play with her with an
interactive toy such as a feather or crumpled paper napkin in a string.
Explain to them that the toy must be put in a safe place when an adult is
not there to supervise because the cat might be hurt if she tries to play
with the toy by herself.
Explain about gentle touching and not grabbing the cat.  If the girls are as
precocious and mature as you describe, they should be able to understand and
more likely to cooperate if you explain the situation to them and try to get
them to see how it is from the cat's point of view.  Find ways they can
relate.
The only concern that comes to my mind is if the little girls are screamers.
I say this, because we have noticed (now that our girls are grown and we are
becoming fogies) that children are becoming louder with less self
discipline. I'm thinking of one family in particular who invited us to
dinner, and their three little girls screamed and shouted the whole time.
It was as if they had no concept of talking.  I hope your girlfriend has
taught her daughters the difference between indoor and outdoor voices.
It is a very good sign that the cat warmed up to your girlfriend.
I'm positive that if you and your new wife are willing to work as a team, it
would be a valuable opportunity for the girls to learn respect for others
and to develop a special and close relationship with a pet.  These are
lifelong lessons that will serve them well for years to come.  I certainly
don't see any reason you should have to choose between your beloved cat and
your new family.
Oh, have the girls visited the cat in your home?  If so, how did it go?
Congratulations, again, to you all
Annie

I agree with everyone on not giving your old friend away.  What a  mistake
that would be.  As for her stiffness ask your vet what would help her. I'm
sure there is some pain relief for your friend.  I have a 17 year old cat
that I have had since she was 6 months old.  I would not even dream of
giving her to anyone.  That alone would kill her.  I do hope you change your
mind on this issue.  Your cat deserves to be happy in it's final years.

Kids can learn a healthy respect for the cats.  If you don't teach
them that respect, the cat will.  Don't blame the cats if the kids get
scratched - children should never be alone with any animal.

An ounce of prevention is the best method.  Trim kitty's nails and
teach the girls that the cat is off-limits.  It can be done, and it
isn't all that hard.  No has to mean NO.

Be sure kitty has her own space.  I would not introduce the children
until WAY after the cat is settled in.  Be sure to give the cat LOTS
of extra attention.  The last thing you want is for her to feel left
behind.

Good luck,
-L.