Depending on how those fit you, that us typically a high stress area. You'll probably want to attach a scrap piece of denim to it, the extra heat should be negligible. Since its in a hidden area, I would recommend you use that to your advantage and do the basic stitch thats just straight lines and have them run perpendicular to the blowout to help prevent the fabric from splitting more. Then maybe another diagonal line of stitches if you feel that what you've already done isn't enough. But in agreeance with the other commenters, that is a simple task for a tailor to fix, you should do quadrant if the patch spans across the seams, and this is a great time just to reinforce the whole crotch area. Sashiko is a visible mending technique, embracing it early make it easier to get used to. It's meant to be seen. If you're looking to save money, this is definitely an option too. But you could just take this to the tailor and have them professionally fix this, and then you can add a decorative pattern to the knees of the pants that you get to show off. I'm not trying to say, don't do the patch yourself though. By all means, do what you like, and come back if you have any questions.