Honestly just buy an old one secondhand. You don't have to use it - it just has to occasionally be near the watch. Put it in your daughter's bedside table or something.

The hardware on the Apple Watch isn't capable of a few important tasks, like installing software updates. It needs to be near an iPhone that it's paired to for those to happen.

I don't recommend using the "family member" feature - it severely limits the feature set of the watch. Instead setup the phone in your daughter's name, with a proper Apple ID that you create for her. Don't give her the passcode to unlock the phone until she's older.

As for losing a phone, iPhones have an extremely robust system for finding lost or stolen devices. It even continues to function (for weeks) after the battery goes flat. And it can pop up a notification on your watch if you leave it behind at an unfamiliar location (leave your iPad at home... all good. Leave it at the airport, you'll be alerted within a couple minutes). This feature alone is probably a good enough reason for your daughter to be all in on the Apple ecosystem. There are other features too.