Hmm, maybe I'm over analysing this post but I think sometimes these issues can be a little more complex than ... oh it's your fault your too soft, say NO and that'll teach them.
I think you need to understand what it is your daughter is wanting and why she wants it before simply saying 'NO'. I would seek to find what the underlying driving force is for the behavour, then once you understand the childs emotional needs you can maybe create the required boundaries and expected behavour while still giving the child the emotional support their seeking.
My DS who at 11/12 is alot older than your DD so not sure how much of my experiance will help, but he had issues where I needed me to sleep in his room, he was just so unsettled with residency going to court and maybe pressures of a new school.
OK so that maybe not applicable to your situation but some helpful stuff at least I found was.
1/ I allowed DC to attempt to express what the issues are but tried so hard not to pressure him as not really able to express what the issues were. Talked about incidents when things were less stressful/emotional, rather than at the time.
2/ Talked about what the boundaries and expected behavour is, again when things are calm and not being controlled by emotions. Then said what would happen if he can't control his emotions and it's his emotions that seem to be controlling him. Explaining that, I was going to TAKE CONTROL, and in what form it would take, and that I was doing it to help.
3/ Then when I did need to TAKE CONTROL I attempted to as best as possible do what I said would happen.
4/ Finally talked about how it felt when I did TAKE CONTROL and if anything could be improved for next time they need help. Either things they could have done differently or things I could do differently. Again some time after the even when they were much calmer and level headed.
... mehh but what do I know ...