The thought of this donation is well intended but converting those bitcoins to Fiat which is just as vulnerable to theft seems questionable at best, especially since it indirectly supports the same state that victimized her and her family. On the other hand can she is only 8 years old and bitcoin may not be user friendly enough for her to handle securely.
Here is an idea.... Can you print off either a multisig paper wallet or SSS paper wallet and enclose it with the 600 USD cash and a letter and I will be happy to donate some more bitcoins to make a starting deposit on that paper wallet. (Will another family member be able to help her secure the shards?) Let me know if you can do this an I'll send over some more bitcoin. It will be great to watch that savings grow for her over the years as well.
As far as the inscription is concerned I think it should be something that instills a sense of solidarity in her fellow humankind. I'll try thinking of a few examples that are pertinent to this situation.
agree that cash by mail isn't very prudent, i prefer you send a money order.
for the transcription , this is slightly corny but something like
'life isn't always fair, but there are forces of good and people who care'