You missed the whole point. The were on the cusp of FG range and that’s why he accepted the penalty. I agree with you if they were anywhere else on the field.
Your son would have been correct if it was in a different part of the field. If the PI call didn’t happen, they would be out of FG range. If you decline it and no PI, it was 3 pts.
Run schemes. We ran basic meathead run plays against UMASS thinking we just bully them. Our RBs were also trying to hit home runs on every touch. Basically, we overlooked them and didn’t take them seriously.