I'd like to use Find iphone to track my teen's phone. Will my child get a notification about another device tracking the phone?

There is no notification for just tracking.

If you make it play a sound, or lock it, they will get notified.

If you know the Apple ID being used for iCloud and find My iPhone on the child's phone, you can log in to the Find My iPhone app on your Mac or iOS device to track it. No permission required.

Otherwise, you can use Family sharing to track the iPhone, but that does require the other iPhone is set to share its location: Family Sharing - Apple Support

Your child won't be notified; however, her/his iPhone must grant you permission to track it.