If you delete it from your devices, nothing, It would still work on her iPad. If you delete it as an account entirely, her iPad will stop logging into all the associated Apple services she is currently using: All the Apple stores (App, Books, Music, etc.), emails, messages, icloud storage, and more.
The best solution is to let your granddaughter make and use her own Apple ID, or if she's not old enough then for her parents to do so. Right now you own everything on that iPad, and there is no way for you to transfer that ownership (as far as I know). Best to take care of that now, with lots of warning, so that she can save anything important she may have on that iPad or in that account's icloud storage.