During a recent bike ride my son went to pedal after walking across a section of grass, and as he did his pedal just dropped out from under his foot. It must have been loose, working its way out, and when he pushed down it ended up stripping out the threads, making it difficult to screw and tighten back in. The following steps is not ideal, but it will work well for a long time. I hope it helps. (Note: I also used this chance to have my son learn how to do some repairs.)
Step 1: Materials
-The un-attached pedal
-Correct size wrench, in our case 5/8ths
-Pick (Since I do not have the tools to re-thread properly with a tap and die set)
Step 2: Cleaning the threads out of the crank arm
Using the pick clean up the inside threads to clear any significant obstructions, it will help also to help where the threads were smashed down to open them back up. Also you will want to pull out the threads that were broken out, be careful there are metal slivers and can hurt.
Step 3: Pedal threads clean up
This step may not be necessary but it can help. Using the small file using the good edge go around in between the threads, and make sure they are smooth, be careful not to take off points of them you will want them to bite in.
Step 4: Screw the pedal back in
This step may take a little bit, it takes some wiggling and working to get the threads started, but since some were stripped out of the crank arm, it proved impossible to align it all back up so we got it started the best we could.