Buy some denim. Buy some heavy duty Heat and Bond. Wash and dry both the jeans and the denim in the warmest/hottest water you will ever use. Do NOT use fabric softener or dryer sheets in the dryer. It will keep the Heat and Bond from sticking well.
Turn the jeans inside out. Put them on the kiddo. Draw a line where the top and the bottom of the inner patch will go.
There are several ways to do this. This is my preferred way.
Take a piece of the Heat and Bond and adhere it to the right side up/front of the denim fabric. Then peel off the paper. Cut the patch to the size needed. Turn the side with the Heat and Bond towards the knee area, let it cool a moment. Check to make sure it's all adhered.
You now have a super strong knee area and it will last a long time.
Even if it gets ripped up you can adhere another patch behind it, on top of the other one.
When I do this I put the right side of the denim fabric towards the back of the jeans so that if the jeans wear a little thin only the right side of the denim patch shows, not the wrong side of the denim fabric. So that's why I iron the Heat and Bond onto the right side of the patch fabric.