If you can match the colour to sewing thread and the holes are small, it can be easy enough to darn them with that. So long as the thread goes through the 'V' of the knitted stitch it will hold it without fraying, and small holes are unlikely to show when mended. Bring the thread through from the back to the front, and mend with the front facing you.
Failing that, depending on the style of the jumpers, you can repurpose them - eg cutting a few inches out of the sleeves and hemming at both ends to make wrist warmers, using a polo as a neck warmer etc. A jumper with cables or pattern on the front could make a cushion, that sort of thing.
To keep the blighters at bay, you could store clean jumpers in sealed bags, making very sure that they are bone dry after washing.