I gave my old Apple Watch to my little sister and it won’t let her connect it to her phone saying it’s still connected to mine even after I erased it

Here are two ways to unpair the watch:

• See the 'How to erase your Apple Watch if you don't have your iPhone' section of:

Unpair and erase your Apple Watch - Apple Support

• With a web browser, go to: https://www.iCloud.com and sign in with your Apple ID. Open Find My iPhone and select All Devices (at the top). Then click your Apple Watch, select 'Erase Apple Watch', and select Next until the device is erased. Finally click "X" next to remove your Apple Watch.

Erasing doesn't remove the activation lock;

What to do before you sell, give away or trade in your Apple Watch