This question already has an answer here:
This is not a question about transferring from one iOS device to another (straightforward and addressed at length in How to transfer Clash of Clans from one iOS device to another. This is a question about transferring from one Game Center account to another. It happens that in my case this is within a given device.
My kid has started to play Clash of Clans on my iPad - under my own Game Center account. Now I have reconfigured this iPad for him to use, with his own account (which I created in order to use the Family Sharing features.
How do I transfer his CoC village from my Game Center account to his?