S.,
I feel your pain as I too have been through this with our daughter. We always put her to bed after she had fallen asleep and she was sick alot so she woke up during the night. Once the second child arrived it became a bigger issue because I was working full time with an infant that woke up to nurse and a 2 year old that woke up wanting mommy.
We started by putting her in her "big girl bed" and I sat in a chair in her room and read her a story. Once the story was finished she was required to close her eyes and go to sleep. I would reassure her that I was there and would stay with her until she went to sleep. (Take a book for you to read as this could be a long process the first few times) Gradually as she got used to the routine I would move the chair closer and closer to the door then out into the hallway. It eventually got to where she would open her eyes discover that I was missing and I would run back to the chair and explain to her that I had to i.e. go to the bathroom, get a drink of water, etc. Eventually she became comfortable with me telling her I was going to iron my clothes for the next day and then I would come in and check on her. Usually she would be asleep when I checked on her.
When our son (the baby) was 18 months he was already sleeping in his crib on his own (we learned how hard it was to make the transition with our daughter) when we moved him to a toddler bed. It took about a week for him to adjust. When he was three we moved him to a twin bed. No problems.
Good luck and hang in there. They're now four and six years old and still occasionally come to our bed in the middle of the night. If I'm awake enough I'll take them back to their room or just enjoy the occasional snuggle and cosleep. But four in the bed at once can be too full even for a king sized bed :). They grow up too soon anyway.