I'm going to take my young niece's keychain.
What do you need it for?
To put my keys on it.
It's not right to steal and it's especially mean to steal from a child. Leave your niece's keychain alone and get your own.
Please, I just want to borrow it. I'll give it back to her.
Even if you plan to give it back, your niece could still be upset about losing it when she finds out.  She might not even trust you to give it back, as kids often pull tricks like that.  If you need to keep your keys together temporarily, there are other items that you can use.