Why does this ridiculous kids in premium cabins garbage have to be rehashed as nauseum? The bottom line is, if someone is willing to pay (cash, miles, certs, whatver) to have their kid in a premium cabin then that kid has a right to be there. If you don't like it, honestly that's too freaking bad. In all my years of flying the number of obnoxious / crying children I've had to deal with is SIGNIFICANTLY dwarfed by the number of obnoxious adults I've had to deal with (both in C or F and Y). I'll take a whiny kid over an obnoxious whiny adult any day. And before anyone starts, I don't have kid so I'm not coming at this from a parent's perspective.
I also want to comment on BJ's statement that switching seats some how disturbs other pax. The solution to this is simple: reserve an aisle seat. If you're not climbing over someone to reach your seat I fail to see how you're disturbing anyone. Is the sudden appearance of a different seat opponent going to shock you to the extent that you will be "disturbed?" I think that's one of the most ridiculous things I've ever heard. I think, however, from an FA standpoint, switching cabins is problematic because its hard for them to know who is really supposed to be there and what's going on.