How do I set up Family Sharing with an old iPad and a new iPad without losing my kid's data?

My kid has been using my old ipad for years, and has made it her own. It's been our only apple product until today, when I brought home a new ipad for me. I'd like to set up family sharing, but am running into obstacles.

1. Her email address is already being used for an apple id. I suspect we set this up years ago and forgot we did it, but as there are no devices attached to her apple id, I cannot find her among the family sharing options
2. Since her email address is already being used for an apple id, I cannot add her to the family sharing via that email address.
3. She has a ton of apps and chat history on the old ipad, under my apple id. I don't want her to lose all of that on the old ipad, but I also don't want it any of it on my new ipad

Because I'll be using the new ipad for work, we desperately need them to not be linked or synced. I'm new to the apple environment, and stumped and not having fun. Please help!

[Re-Titled by Moderator]