How do I repair the crotch of a pair of jeans?

If they're only ripped at the seam, like the seam threads are torn. ... Turn them inside out and stitch first one direction and then the other on a sewing machine. You can purchase needles specifically for denim. Or, taking them to a dry-cleaners who offers alterations might be necessary if the fabric is really thick.

here are ideas/videos https://www.youtube.com/results?search_query=diy+repair+blue+jeans+crotch

Sharon is right that you most likely will need to add to or reiforce the seam allowance first before resewing the crotch seam back up. Otherwise it may come apart again very easily if the original seam allowance is frayed, or disintegrated.

Is the crotch worn out or has just the stitching in the seam come undone?

Here's a couple of videos on how to us a sewing machine to mend a jeans crotch.

https://www.youtube.com/watch?v=w3uYH3uIFqY

As Toni mentioned, for best results when working with denim, use a reputable brand of sewing machine needle marked Jeans, or Denim.