C.W.
We co-slept with each of our children (5)and sometimes 2 at a time. The easiest way that we found was to get them their own bed with the bedding of their choice (cartoon character, transformers, cars, etc.) decorate their room and tell them that it will be theirs when they are a big boy. After a couple of days we started "bugging" them while they were in bed with us ( getting a little too close, pulling off the covers, nudging them as they were about to fall asleep) then telling them oops sorry, your getting SO BIG. We did this with each of our boys less then a week and they ended up getting out of our bed and going to their own room and thinking that it was their idea. That way they didn't think bad of their sibling who stayed in the bed or a new child who come into the bed because it was THEIR idea to leave in the first place.