I want to give my niece some bitcoin, but she has to be 18 to have a Coinbase account. Is there a way for a 13-year-old to sign up somewhere for bitcoin, or is it strictly for over 18-year-old's? Good to get them familiar with this from a young age!
You can create a wallet for your niece. We do not need to send bitcoin to the coinbase, there are many different bitcoins. Or you can send it to Binance trading platform and give your account to your niece. When your niece wants to sell it, it will be faster instead of transferring the bitcoin from the coinbase to the trading platform.
This is not a good suggestion at all. You just pretty much suggested a different exchange. It looks like you also need some research and reading to do.
Also, "there are many different bitcoins"?
Having a paper wallet is pretty secure, bad thing about it is that it's a paper and getting wet or being torn is it's disadvantages and in order for her to withdraw the funds in the future,
Having a paper wallet is only secure when it was actually made in a secure manner.