Is it safe create a a peper paper wallet from bitcoin-qt n dumpprivatekey?
Yes. (Modern versions are called "Bitcoin Core", not Bitcoin-Qt).
if I sweep one private all other ones that I have in cold storage
If your Bitcoin Core is running on a computer that was ever connected to the internet, then the paper wallets you create are not "cold storage", they are just paper wallets.
will be vunerable because they are derived some way from the one spent?
Then don't sweep that private key. You have a few options...
- Sign your transaction offline so that the private key is not exposed.
- Generate a new Bitcoin Core wallet for each paper wallet that you create
- Sweep all private keys that were generated from the same Bitcoin Core wallet. Send any excess to a new paper wallet.