Like any transition, it takes time.
Now, this is what your daughter is adjusting to.
My son, is 2.5 years old and still sleeps/naps in his crib. Yes, he can climb out and open doors. But, we just tell him "no...it's dangerous..." and he has stopped doing that. He still sleeps very well in his crib. He likes the routine and he just loves his crib. He still fits in it as well, so we have not taken him out of it, yet.
Meanwhile, just at night, we do let him sleep with his sister, on a floor futon we have in our room. I also co-sleep with them as needed. But, yes, my son, at his age... will also get up, open the door, and come out. He thinks it's funny. But we don't punish for it... it's a phase. All we do, is verbally coax him to "go back to bed..." or we tell him "in one minute, go back to bed..." and then he does. We have found that this is just his way of winding himself down... and it's a "routine" for him... he likes to say "good night" to us this way. Yes, it's repetitious sometimes... but he does go back to bed, without yelling or tantruming. We just keep the rooms dark, no lights, no extra stimulation, and we make everything QUIET.
Then, also, in my daughter's case... when she was a toddler, we too needed to be with her as she fell asleep in her own room/bed. But we were flexible with it. We did not "lock" her in her room. We let her come to us if she needed too. We even let her come sleep with us if needed. AND the thing is, at this age... they DO develop night-time "fears' and fears of the dark etc. AND they get "separation anxiety" too. It's all developmental based, and they have night-mares too at this age. So, as Julia M. said, they need us and putting locks on doors scares them. It turns the whole sleep routine into a unpleasant event. When it should be comfortable and relaxing.
You either let her transition to her "new" sleep arrangement as she is able too... or you put physical boundaries around her and things that seem like "punishment" for a child...
The thing is... your daughter is having anxiety about it all... ie: waking, crying, wanting you near her etc. There is a big difference between a crib and a bed, for a child. She was fine in her crib... perhaps, try and verbally 'teach' her about staying in it. My son still does, with our verbal coaxing and it is no longer a problem.
But well since she is now in her own bed... I would recommend LETTING her adjust to it, in HER own pacing and ability. Remember, a young child does not have "coping skills" yet... and this is why they act this way. Then, you simply explain to your 8 year old, that the baby is having a hard time adjusting to everything, that this is normal, and that she may cry sometimes at night. So don't worry. FOr example, we explain things to our eldest daughter this way, regarding our 2 year old son, and she understands. We simply explain to her about development in a young child... so that she can understand it at her age, and not get irritated about it. And, this works for her and us.
You can't "make" a child stay in their bed like a statue... unless yes, you lock them in and don't answer them and let them cry. THEN after a time, they will give up and then tire themselves out and fall asleep. Thereby "learning" that their anxiety/calls to their parents will not work. Then they stay put, out of the fact that it gets them no response or comforting. This is just one method. BUT bear in mind, that there will be hiccups and regressions along the way. Which is normal. What then?
All the best,
Susan