Can I merge 2 Apple ID/iTunes accounts then change 1 to a teen account

Finally making the switch from Android to Apple. Never before an Apple user. My daughter uses one of my old email addresses for her iTunes and Apple ID account, she is 14. I stay logged onto that email for her iTunes account and some of my other contacts. Unlike previous questions on here, I do not want to create a new child account, because from what I've read apps and items purchased will not transfer. I want to know if I create a new Apple ID / iTunes account using a new email can I connect or merge the two accounts and change hers to a child/teen account? If so, how? If not, any advice or thoughts on what I should do or not do? Should I just leave it as is and monitor her account as usual? I do not want her to loose all her purchases.

Apple IDs cannot be merged.

You can add an existing account to a family sharing group. Purchases made by any account will be available to everyone in the group, so no purchases will be lost.