I think Low kicks are best.. the higher the kick, the longer it generally take to travel and the more likely to have a knife used in the 'block' .. preferably against the soft tissue of the inner thigh.

Also the bigger and heavier the knife the less and less kicks should be used as the result of failure would more disatrous than with one the size of a tactical folder, IMO.
The use of the legs should not be ignored in any combative training.