We coslept (by choice) since infancy and when DD got a little over 4 I made a big deal about her new room, decorated it, made it very special and then moved her in there. We talked about it in advance. She woke up for a week or so and would come and get us "for a cuddle." We accommodated that and walked her back to her room. She also has a flashlight, one of those turtles that makes stars allover the room and lots of stuffed animals.
I didn't get mad at her but I did keep reminding her that we need to sleep and the waking thing went away after a week or so. After 5 mos she had a gi bug and I had her in my bed for a few nights (easier to be there with the bucket when right next to me). When she was better she went right back to her bed, no issue. She does occasionally wake my DH "for a cuddle" in the middle of the night but when he is away (travels a lot) she doesn't wake me, so I think it's a case of what the parents will put up with. I just kept telling her she was not to wake me for a cuddle (not in a stern way, just matter of fact) so she doesn't do it with me.
I think what was helpful was making a really cool room for her, making it a big girl thing, talking about her friends who slept in their own rooms and keeping my cool about it, not engaging in a battle over it.