D.M.
There are all kinds of appliqués available at stores like Joann’s. Other sources would come from fabrics that you put some wonder under on the back of and then fuse to the pants. There are so many very cute fabrics of every theme and cartoon characters form the internet fabric stores too. I used to put hearts on my daughters knees. One thing that will probably help, if your fabric is stretchy your patch should be similar fabric. In this case you would do better not to fuse it, but turn the edge by sewing it to some wash away stabilizer first. Cut a hole in the stabilizer and turn the patch right side out. Sew the whole thing onto the knee as is. Then wash the garment and the stabilizer will disappear. Another tip is to put a patch on the knee before the pant is worn, giving it a double knee before it gets worn out. You get a lot less stretching out this way.