L.,
The thing I found that worked with all 3 of my kids it to take them back to bed - every single time. Be consistent with this. Pretty quickly they get the idea if they come to bed with you or grandma, they will be put back to their own bed. Yes, your 5 year old will try to reason with you, but don't take the bait.
This idea came from SuperNanny. She also said:
1st time - give kiss/hug and say, "it's time for you to go to bed.
2nd time (same night) - just say, "bed time"
3rd time and each subsequent time - just take them to bed without saying anything.
You might want to verify the wording, but I'm sure it's pretty close. Here's the book info:
http://www.amazon.com/Supernanny-How-Best-Your-Children/d...
Best of luck.
K.