That's one of the easiest things to do, really! When your kids outgrow their pants you don't have to say goodbye to them.
A couple of weeks ago, my cousin was going through her daughter's clothes (who's 8 months older than mine and so, mine gets all her clothes when they don't fit her anymore)... and she realised she had way too many pair of pants.. and so did mine too.
What to do? Simple, right? Cute a pair of them into shorts, hem and that's it. I used a double needle stitch on the hem to keep it looking cute, and because the fabric had a bit of elastic on both.