I don't think knee-high is popular with young people these days. She might prefer over-the-knee - I see the stretchy, flat kind a lot:
https://www.nordstrom.com/s/stuart-weitzman-reserve-over-the-knee-boot-women/3024784?origin=category-personalizedsort&breadcrumb=Home%2FWomen%2FShoes%2FBoots&color=005
Those are basically the OG but there are cheaper versions.