If they are stretchy, you can use the fusible knit interfacing (it comes in black and white). If it is a rip or just reinforcing, then just use a large piece on the inside. If it a hole, you will need to get a bit more creative if you don't want the white or black to show through. Several of us at work use the black fusible knit on the inside of our work pants. We make sort of a 'saddle' over the inside leg/crotch to solve that thinning chaffed area. Make it large enough so that you can run a stitch along the edge at the leg seams. This doubles the life of the work pants. You can also use this on t-shirts that get that tiny hole. It usually can't be sewn around because it will show, but it will withstand many washes and the knit is soft on the skin. I have started to use it when repairing the knees of the kid's jeans because it is so much softer on their skin than the twill or denim patches.