Can a chiropractor fix knock knee?

This will depend on many things, most importanatly the cause of the knock knee (genu valgus). If the condition is in a young child, it will probably resolve on its own as the skeletal system matures, but it is advisable to have your chiropractor monitor this. If it is caused by pes plantis (flat foot) then orthotics can sometimes help. Some conditions are not repairable short of surgery, which usually isn't recommended unless there is severe pain or loss of function.

Your best bet would thus be to seek the advice of your chiropractor to have an exam.