When my dd was 5yo, I found this difficult as well. Everything I tried to introduce, she seemed to already know. It took us awhile to find the spot where she was challenged and learning new things. I found that place by compacting curriculum and moving very quickly for about a year (as in going through almost 3 grade levels of math). It was difficult and a little scary at the time because I was using her brothers as a reference point and getting a little panicked at how to keep up with her.
We did find a balance point eventually though there are still times when I feel like I am being dragged behind her because she tends to learn whole to parts so while I am trying to introduce something, she already has it figured out and is ready to move on. I am used to her now, though, and don't "panic" anymore.
Like someone else mentioned, I find it best to use a high level spine and plan to go very deeply into subjects like history and science. I have the materials on hand and dd is nearly always very interested in learning more so I am prepared to go there with her. She is 9yo and currently going as deep or deeper into subjects than her brothers in middle and high school.
