When I read the book, I thought that the transition was well done. It was a long process that was intricate and it tore Anakin apart. Thus, when he killed the children, it was accepted. That part of the movie, in my opinion, was done poorly. It was rushed and as you said, he agrees to be Vader, the light switch is flipped, and he's killing kids.
Yeah, I thought Order 66 itself was rushed.