In current fashion, it is most acceptable for a little boy's or girl's jeans to have a soft, well-worn, knees in shreds look but there are still times when the dress code requires the nice and neat look. It doesn't take a lot of experience with a very active child who is just beyond walking into running to know that a pair of good pants can die in tatters after just a few minutes in the playground. Little boys are never able to outgrow their pants. Sometimes it's a baseball game, coming down off of a slide or ride or out of a tree. They just don't last long.
One of the advantages of taking four years of home Economics was becoming very familiar with the fabric store and all the notions available besides the patterns and fabric. When my son was about three, I noticed his jeans wearing out much too fast. The invention of iron on patches was wonderful. Just don't put them on the outside of the pants. When the pants are brand new and never been worn, turn them wrong side out and follow the instructions for the patch and the fabric to seal the patch securely. There are different weights of patches to match the weight of the fabric of the fabric being strengthened. You don't even need to tell the child wearing the patched jeans; it will be like they came that way. You will notice with time how much longer the pants will wear but don't scrimp and buy a cheap no name brand of patch. Go with the quality and pay a little more. The patch won't start peeling up at the edges. I buy several different weights in a pack of assorted colors just to have plenty on hand for repairs.