Son iPad doesn’t work because of mom new phone

So when my daughter set up her son iPad for she accidentally set up two profiles up for him. She created profile under her iCloud account for him (this was wrong way so apple had her set up one correctly in his name) So the active account, the correct iCloud account is under his account name and his iCloud name, but it is tied to hers. She got a new phone and in switching the iPhone to new ones it move her sons iPad to the inactive account under her name. How can you move it to the active iCloud account so his iPad will work again.