Praise G-d. The investigation ended as quickly as it started. It was a mere formality - The school had to report what happened, but made it clear to the investigator that they weren't concerned. We've done a good job keeping them apprised of his behaviors at home and we also notified them as soon as he got hurt the first time. The investigator made it clear she wasn't concerned as soon as she walked through the door. So all is well. They didn't even really speak to Big Guy, as when they stopped by school he refused to show them anything or talk to them, and she didn't think it was worth trying to convince him to talk to him. So we just answered some questions, she was very nice and said we'll get a letter clearing us within a few weeks. I have to be diligent about contacting CPS to put us back on the list when we're cleared, in case Little Guy re-enters the system... because while you're being investigated, your home is "closed" to all new foster placements.
It was painless, other than the waiting. I really appreciate how respectful and friendly the investigator was. She apologized for it taking so long to come out - She was out of the office right after our holidays ended. I am just so glad Big Guy was on good behavior when she came - it would have been stressful to have her looking on as we handled him punching or kicking, though I'm sure it would have made a point as to how he got hurt in the first place! He was great tonight, even if he wouldn't eat the chicken shnitzel I cooked special for him.
The morals of the story:
1) If you think an allegation can't happen to you... It can. And it will. We are educated, middle class folks* who are completely anti-corporal punishment, have child-positive values and treat our kid with respect even when we're being physically pummeled by him. We don't have neighbors that hate us. We have a great relationship with (birth) mom. We have great relationships with all of the professionals involved in our foster kid's life. Big Guy hasn't made false allegations per se. We've documented everything a lot and are quick to call CPS if he ever makes a threat of some sort or his behavior is particularly difficult or he gets hurt. We've done (almost) everything by the book... and it still happened. But guess what? It's also not always the end of the world. I will be a lot less freaked out if (when?) this happens again.
2) If you ever injure your child by accident, there is no such thing as documenting it too much.
3) Having open lines of communication with your foster kid's school social worker, school nurse, and teacher is really helpful if/when an investigation happens.
*This is not to say educated or middle-class people don't abuse their kids - Instead, I'm referring to the fact that we are privileged within the child welfare system. My non-scientific observations are that middle class, educated people are less likely to have allegations made against them and more likely to have them dismissed. We've already seen how the foster care licensing people didn't even look at anything that was on their safety checklist because once they saw what was clearly a nicely decorated, white, middle class home they seemed to lose interest in checking the bath water temperature, the fire and carbon monoxide detectors, etc.