IME there's no real way *for you* to stop the cracking once it's started. Sometimes it'll stop spreading on its own, sometimes it won't.
IF it's in the plastic then you're simply looking at taking it to a shop, simple as that. Some shops can fix the plastic, others can't and will have to replace the bumpercover.
If the crack is only in the paint, you might try some touchup clear, but I think it might still need professional attention at some point if it keeps spreading.
OTOH, I have a small stress crack in the paint of my MPV's front bumper cover (probably from something like your experience, dunno how it happened) and it hasn't gotten any worse over the last three years or so. It spread for a little while and then stopped and it's minor enough that I just live with it.
Sorry to say...I think that in the end it'll either stop spreading or it won't and you'll either be able to live with it or you'll simply have to take it to a shop and have it repaired. While it was a very minor accident, it was still an accident and, well, sometimes that's how it goes.