K.Y.
We have bunk beds for our 3 and 5 year old. Our plan was similar to yours, 5 on the top and 3 yr. old on the bottom. Day one they switched on me and refuse to sleep in the correct bed. So far no problems. There is a good size rail going all the way around the bed so that helps. My 3 year old climbs down every night to get in bed with his brother with no problem. He has always been a climber though. You might try One Step Ahead, it is a catalog that has great safety stuff in it. You could also try putting their names above the bed you want them in and different blankets on each bed that is specific to a character or something they love. Best of luck to you!