Hey,
I don't know much about Down's Syndrome, but have a slight disability that effects the way I walk, I also suffer from knee and leg pain. Your daughter might benefit from getting orthotics, if you can, visit a private orthotist who can make custom orthotics or footware for your daughter by looking at her gait and feet. Physiotherapy would also be of benefit to help strengthen certain muscles, especially if she has hypotonia (low muscle tone), and get your daughter's weight back on her heels. You should be able to get this on the NHS, if you visit her GP to get referred. If you want a private physiotherapist, they can usually be found on the web, they will book you in for an assessment or consultation. I don't know what type of physiotherapy would be beneficial for your daughter, but make sure it is a pediatric physiotherapist, as they work with children.
This site might help a bit in finding a physiotherapist
http://www.physiodirectory.com/index.php - if you select the county that you live in, it should show some physiotherapists in your area and the expertise they have.