I used the sandbox pattern a lot for my son prior to the field trip pattern. I found that on the whole my rough and tumble son and his trousers came out pretty well unscathed. Except a few mishaps wheat the fabric ripped through movement in a not seam area. I think I lost s few pairs that way just out of the constant washing of the fabric. His school pants though have stood the test and after two year only one pair is just starting to ware.
I am yet to try the field trip pattern out but am looking forward to using it. I think it will be a fun pattern to use and also with the knee, it will help them to last longer. I like to use track suit fabric to make them for winter (sandbox that is) and I have made a couple of pairs in denim and they have done really well. Something that has surprised me is the corduroy as they were the ones that ripped in the unusual places. Mind you the quality was fabulous so that may have been a reason to explain it.
For the Field Trip I think making them in denim would be fabulous. You could maybe try a chunky corduroy, possibly track suit fabric if you were going without the pockets and my first choice after denim would be drill / twill or maybe canvas? The fabric will give a different look and will also help with the longevity I think.
Hope this helps.
Tamara