Can't say my DD has similar scores (her CogAT scores were terrible), but she is the same age. Luckily she did much better on the WISC than the CogAT. If your DD's scores follow my DD's pattern of doing much better on the WISC, she will get a perfect WISC score. LOL. DD's WISC GAI (nonverbal and verbal scores) was 33 points higher than her CogAT composite. What I told her mostly had to do with what I knew might be issues. For instance, she doesn't need to be shy or give one word answers. The psych told me that she was not the slightest bit shy or quiet and was overly verbose at times so maybe that was counterproductive! I don't know. I also told her that for any section that is timed, to work as fast as she could and don't worry about being sloppy. This may be a bad thing to say to some kids but not to my DD who is incredibly slow. She still bombed the coding section and when the psych showed it to me it showed perfectionistic handwriting frown. If I hadn't said anything then how much lower would her score have been!
I didn't tell her much about "why" we were having this testing done and luckily she didn't ask. I'm not sure what I would have said. I think if it's in a school setting, your DD won't think much of it because pull-outs for assessments happen all the time in school. Good luck! So stressful--since I was right there in the waiting room I was on the edge of my seat. Esp. since I could hear some of the questions!