Toddler in her own bedroom.

My daughter will be 3 on the 24th. I also have a son who will be 4 months on the 20th. My boyfriend and I live with his dad right now, but she has her own room. However, she still sleeps in bed with us (which I honestly don’t mind.) I would like for her start spending time in her room, but she’ll only be up there if I’m with her or my boyfriend. She’ll stay in there by herself if the door is open, but she always runs in our and I’m afraid of her getting into the bathroom or my boyfriend’s dads room.

Does anyone have tips on how I can get her used to being in her room by herself with the door closed? Or ideas on how to keep the door open, but with her unable to get out?