Assigning new device to child's account
You can do this either way, you can use your account or your child's, but the steps to take to set up Screen Time will vary depending on which one you use.

It sounds like the iPad is for your child to use primarily, in which case I would recommend using his/her account, since it makes it more streamlined and intuitive to use the parental management features. (usually, that is) It is also the only way to take advantage of features like ask to purchase, etc.

The Apple ID that is used device-wide, i.e. for the iCloud features, iTunes/App Stores, etc. should be the child's account – the top of the Settings app should display your child's name and account for the Apple ID settings menu.

When you set up screen time, or other parental management feature, a login prompt will come up, probably somewhere around when you set up the management passcode. Here, you need to log into the guardian or family organizer Apple ID.

But you can avoid this confusion by configuring Screen Time for your child's device from your own Mac, iOS device, or iPad, if you have one. Turn on Screen Time in family sharing and it will allow you to manage your child's device from the Settings app or System Preferences on your own device remotely, kind of like a cute mini version of an MDM deployment :)