How long have you had the pants? If they're as old as I think they are, it may just be time to bite the bullet.
Otherwise, kevlar patches with matching fabric over-top can be sewn onto the knees. Fuse the cloth (ie. with that tape stuff or something similar) and then make it at least 1" larger than the hole. I'd go right to the seam. That way you have the fused part and you can "tie into" the seams on the pants. If you need additional stitching, you can add it.
I've always consulted an expert seamstress--my Mom.
__________________ MCB Feedback
Please ship to me using US Postal Service when shipping to Canada.
Don't ship to Canada? No worries--I can work it out with you: US Parcel Pickup