Ok, so if it is not a rule, how is anyone to know that taking a child's picture could cause you to have a negative interaction with a CM?

You are making Disney sound like a police state. I don't for one minute believe that CMs can arbitrarily decide who stays and who can be kicked out of the parks, just on their mood. Which seems to be what you are saying. They could walk up to grandpa and say Hey, I don't like the looks of you. Leave.
Don't the CMs have a list or guidelines to follow?
And is there a difference between what would cause Disney to refuse admission and what would make them eject someone from the parks?
To answer your questions:
refusing admission = ejecting them from the parks, the only difference being where the refusal happened. If they refuse your admission while you're in the park, you have to leave.
I'm sure the CMs have guidelines to follow, and I'm sure we'll never see them intentionally. I'm also sure that any questionable behavior that may result in a guest's ejection would be brought to a manager. The reason:
- they are better trained and have a higher confidence level from Disney management than a front line CM
- they are fewer in number, thus more consistent in the overall approach.
So what keeps Disney CM managers from running amuck with all this power? The fact that Disney is a business. If word got out that random people have been kicked out for no apparent reason, Disney's less likely to sell tickets tomorrow. It isn't a sustainable business practice. So they are trained, and one of the ways to really weed out the bad guys is to let a guest make the wrong 'right' before being kicked out.
in the worst case scenario of a guest being falsely accused, that guest would likely agree to delete any photos that appear out of the ordinary. Meanwhile, the manager is getting a good look at the overall pattern of those photos - and just as the supreme court defines pronography, 'they know it when they see it.'
Sorry it isn't more black and white. But again, if you were a guest that was somehow violated by another guest... the LAST thing you'd want to hear from a CM is "sorry, what they did isn't technically against the rules, so there's nothing I can do."
Disney always reserves the right to do something as a protection to vast majority of guests who aren't bad guys.