I would be better safe than sorry and assume she won't be able to do most if not all rides with warnings. The one exception would actually potentially be Soarin' if she's not showing signs of vestibular system problems. (I know a previous poster said she had problems, but I suspect that's one of the places where it's going to be incredibly different person to person.)
I had Chiari decompression surgery along with a c2-> skull fusion just over a year ago at the age of 26 (all part of one surgery, not just to treat the Chiari). I also had a surgery on my lower spine the day before so I am assuming my overall surgery experiences were more extensive than your daughter's. I also have a genetic connective tissue disorder (Ehlers Danlos Syndrome) so I'm guessing I'm overall more complicated than she is. That said, as an adult it means my skull has at least stopped growing and the fusion means my skull shouldn't slip down to cause similar problems (my Chiari was partially such a problem because of the EDS) in the future.
Pre-surgery I cleared my schedule for pretty much 3 months afterwards. It turned out that I went through the initial round of healing pretty quickly so at the 3 month mark where I figured I might be starting to drive, I was at a conference in Orlando (although not at WDW) that I hadn't expected to be able to attend. At that point I don't know that I would have felt comfortable going on *any* ride at WDW unless I could be in a well padded wheelchair with good back and neck support during it. At 4 months out (so about the time lapse that your daughter will have) I actually started having a return of some of the symtoms that had gone away post-surgery (we don't think they were from the Chairi now as we've controlled them through medication, but had no clue at the time they were coming back). At a year out, I'm glad I had the surgery and I think it has improved my life, but I don't ever expect to be able to ride a roller coaster again (and I loved many roller coasters). I'm sure my neurosurgeon would say no based on the EDS alone so I'm not going to say that your daughter's surgeon is going to say no forever, but I think at 4 months out you need to plan for him to say nothing, but the most gentle rides. You may also need to plan that she'll have less energy than a normal 8 year old because her body will still be doing some healing.
I know I've painted a not great picture, but hopefully I'm giving you a worst case scenario, rather than what your daughter has to deal with. Since I was an adult, my experience may be completely different. If you have any contact with parents of other kids who've had Chiari surgery, you may want to talk to them. Facebook may have groups that could give you advice. (I know that there's a Chairi in Connecticut facebook group so if there's something as specific as for people with Chiari in a single state, I would expect there to be a group for parents of kids with Chiari). If by some strange coincidence your daughter happens to have EDS, feel free to PM me and I can suggest some groups that deal with head/neck surgeries in people with EDS and some of the members have young children who've had Chiari surgery (the kids also have EDS).
Sorry for the negative sounding post, but having spent 20+ years where we didn't even know I had Chiari and then the time between diagnosis and surgery (which was actually fairly short, but at that point I was feeling pretty desperate for relief), I can say that your daughter is far better off having the diagnosis and the surgery now as it should hopefully save her years of pain and neurological problems down the line!