Fortunately, it sounds like your parents only had one violent incident in the past with each other and none after that. If that's the case, even if your ex brought the issue up and the judge believed it, it would have very little negative effect against your parent's character and fitness as guardians to your child. If he brings it up, have your parents explain that it was a sole isolated incident that never happened again and that they've been together ever since the incident and have learned and grown since it occurred.

However, I highly doubt the judge will view it too negatively. Judges are looking for repeated patterns negative behavior in determining who isn't fit to be a guardian. Isolated single incidents usually don't hold much sway. Good luck.