Spyware on ipad

1. Unless the iPad was jailbroken, there is no way to install any sort of spyware on it. And even if there was spyware on it, it would only server to maybe log what is typed or done on the iPad itself.
2. Where are you seeing this other device? If its in the device list in Settings->Account(top of settings), that just means that that computer is signed in with his Apple ID also, which would be a perfectly normal thing to do, and does not in any way give it any access to anything you do.
3. Since the iPad is signed in to his Apple ID, messages sent and calls placed be shared, though unless there is another Apple device using the same Apple ID, this would not be an issue, as there is no way to get iMessages or Facetime Calls on Windows.
4. You will not be able to remove his Apple ID from the iPad. If you attempt to restore it using iTunes on a computer, and Find My iPhone is turned on it will get Activation Lock and only the father would then be able to unlock it.

If you want to uninstall something,

Press & hold on an app until it starts shaking. Then once it starts shaking, release you’re finger, then you’ll see a small cross at the top right hand side of the app. Tap on it, then tap delete

(Hopefully this stops the spyware)

Raz

No one know what your ex has done to the device. I have an easy solution if you're worried that he may be spying on you... return the iPad. Buy a new one for your kids.