First, I think there's some risk if she's telling people you two are dating. Definitely tell your wife, but you should also probably talk to her parents about it.

1. If they start hearing stories about their daughter and you, they already have context for it
2. You can work with them to decide how to proceed

This is an opportunity to teach her about boundaries, appropriate behavior, and unrequited love in a relatively controlled environment. She will listen to you in ways that she won't listen to parents.

You could have a direct conversation with her about how it's not ok to tell other people that you're dating. Tell her that it makes you uncomfortable that she knows you have a wife but still told you she has feeling for you. Tell her that you are happily married and not interested in dating a fourteen year old. Model clear communication.

And empathize with her that it's hard to have feelings for someone that doesn't like you back. Talk about how you've handled it in the past. Tell her that it'll pass. Tell her what she should know as she grows up.

She's a teenager, so her feelings for you will go away regardless, but I think you can make the rest of her life tangibly better by having a real conversation with her.