A baby turtle shell is small, and may be harder to preserve, since the
bones are not as calcified. I remember researching this once for myself.
Here are the options I remember. Note that most of them are gory, and I
eventually opted for enterration and luck.
There is certainly nothing wrong with your daughter's wishes, and it
won't make a difference for the turtle. (You don't know the things that
my poor husband sometimes has to tolerate in my fridge...)
* Professional curators often use several kinds of acid solutions to
remove the soft parts. This looks and smells rather bad, and I decided
it wasn't soemthing I wanted to do to an animal I had known and loved.
It also taeks some expertise to dose/time it right; and working with
acids is not without risks.
* Simmering long enough will eventually remove the soft parts and leave
the shell intact. You will have to deal with the smell (won't be bad
though, unless the animal is already decomposing), then dispose of
the cooked remains. The advantage is that you have a good chance of
getting an intact shell.
* If you have a vermicomposting bin or a compost pile, bury the turtle
there in a spot you can find again. Check every couple of weeks. This
may damage the shell.
* Just bury the animal without anything around it in a spot that is kept
moist all summer. Check once a month or so on the status of the shell.
This may damage the shell.