So my son is 11 and some of his friends have phones and he wants to be able to message them. I let him have an old iPhone 5s we had sitting in the closet, and I set up a child account under my iCloud account. However I cannot figure out how to get iMessage to work, I keep getting an activation error. I have searched and seen that you used to need a SIM to get an activation code but some said that was no longer need after iOS 12. I have iOS 12.4.6 installed and still get the error so what am I doing wrong?