As much as I detest child molesters, Mr. Wheelchair screwed this one big time.
Go take the kid to the cops, Child Protective Services, a Magistrate, and tell the story of what [allegedly] happened and let them proceed from there. Apparently the story was believable because once the state police heard the story the [alleged] perp/perv was arrested. There was no need/reason to make a citizen's arrest - other than emotion, pride and anger.
I don't see any reason to let "grandpa" off on the assault charges, but would prefer to see him do a combination of house arrest/community service rather than impose the burden of a parapalegic on the correctional system.
stay safe.