My almost four year old dd won't go to sleep unless DH or I are upstairs with her. We don't have to be in her room, but we do have to be in ours. She usually goes to bed around 8:30, plays in her room for an hour (sometimes two). This has been the routine since she got her big girl bed in January. At first we had to stay in her room with her, so this is somewhat of an improvement. With a new baby due in October I need to break her of having one of us upstairs and I have no idea how to do it.
Re: DD won't go to sleep w/out parent upstairs
In your shoes I would start preparing her for a change in how you do bedtime, starting earlier in the day "DD, tonight, when you go to bed, Mommy and Daddy are going to go back downstairs. You'll be just fine, we'll be right downstairs if you need us," etc. I'd mention it a few times to really root it in her mind and allow her to ask questions or express concerns and address them - reassure her that she's a big girl and she'll be just fine and if she really needs you, you're right downstairs.
If you aren't up there I assume she cries? I'd let her cry for, say 5 minutes, then go up and comfort her and try to get the reason behind the tears - angry you're not upstairs, afraid something's going to happen, etc. Calm her, but do it quickly, and then head back downstairs. Wait 10 minutes and then have your husband go up to do the same. Wait 15 and go back up to do it yourself.
If it were my daughter, she'd go to sleep eventually. I can't imagine a child who just won't fall asleep out of exhaustion at some point. It may be upsetting and difficult, particularly for the first few nights, but if it's something you really want to change, it may require that amount of work.
How does she know if you are upstairs or not? Could you leave your light on or something to make her think you are and just as some reassurance? Could you leave the hall light on too?
There are times when my 3 year old has been scared of things and my husband and I come up with ways to combat it. She was afraid of bears so we gave her a flashlight. My husband also told her he would stand outside all night so the bears couldn't come inside. He just opened the front door so she could hear it and pretended to go outside.
We just talk to her and try to reassure her or come up with things to make her feel safe. This is a magic stuffed animal that will keep you safe. Or, we will come back upstairs in a few minutes to check on you. I will leave the hall light on. Just come up with a few ideas with your dd on how to make her feel safe without being trapped in your room.