My first thought was that the trousers are old enough to surely get some new ones. My second thought was mending would be less traumatic and my third was I managed to throw away some moth eaten underpants two weeks ago and I better not push my luck!!
Exhibit A the holey pockets...
No pocket lining fabric to hand but I did have some quilting cotton which whilst not ideal will hopefully outlast the rest of the trousers.
Chop off the offending part of the pockets. One was worst than the other but I removed the same portion from both.
Sew french seams.
Sew the binding back around the pocket.
Apparently this was a better job than expected. I was just supposed to fold up the damaged part and sew over it. Left to his own devices this job would have involved a stapler. Not quite sure how you would have used the pockets afterwards. Anyway when they enter the next stage of their life I am sure you need pockets in your gardening trousers too.