Hey J.-
what are the age differences between your children? the problems with her siblings may be due in part to the age differences. I know that at 7-8 years old a child begins to get cognitive reasoning skills - a mind of their own with the ability to tell right from wrong. It sounds to me that your little girl is wanting attention and is seeking to get it even if it's bad attention. I don't have an 8 yr. old, but I was one, a very nasty rotten tempered one at that, and even if I said I hated my parent, I didn't, if I didn't want to leave a place, it was because I was having a good time and didn't want it to end, and I argued because I really didn't understand the "why" behind what was being asked of/told to me - maybe you could start a ritual with her, do something special with just her that you both enjoy every day after daycare and make a promise to yourself that you'll do it no matter what - find a good time of the evening to fit it in to your busy life, that will make her anticipate being picked up by you and make her feel like she's not losing out if she leaves, also it will give the two of you a time to bond, and it will show her that you're making time just for her. And when you talk to her try explaining in kid terms WHY you want her to do something or WHY you believe something, and give her room to disagree by saying that it's alright to not feel the same as you, but that she needs to trust you, if she still battles, ask her WHY she doen't trust you. I made a point when I was 8 years old to remember what my family was doing right and what they thought they were doing right but weren't, I promised myself I'd remember these things when I became a parent, so not only is that where my perspective is coming from, but the part about this being a phase - she might stay solid in her beliefs if something doesn't change, because at 8 years old I also decided on a lot of other things, like that I would never smoke cigarettes, and I won't ever - point being, you're right to worry about this attitude sticking - but discipline isn't the problem most likely - miscommunicating, a lack of trust, and wanting more attention/praise/acknowledgement seem to be her problems, and that doesn't mean that you've done anything wrong thus far, she's probably just struggling with normal kid things but needs a bit more than most kids. I hope that this helps, let me know.