Does a one-time act of groping a child of 13 years through their clothes come under sexual assault? What impact does it have on the child’s mental state?

Presumably you’re not a rapist and/or child molester, correct?

Then why are you looking for the line of “acceptable” and possibly nonprosecutable sexual assault?

Here’s an idea. Don’t look for that line and don’t try to get away with anything. That’s what sexual abusers and rapists do. Trust me, people who act like the person asking your question rarely limit their predation to “one-time” events.

Pooling data from four samples in which 1,882 men were assessed for acts of interpersonal violence, we report on 120 men whose self-reported acts met legal definitions of rape or attempted rape, but who were never prosecuted by criminal justice authorities. A majority of these undetected rapists were repeat rapists {my emphasis}, and a majority also committed other acts of interpersonal violence. The repeat rapists averaged 5.8 rapes each {my emphasis}.  The 120 rapists were responsible for 1,225 separate acts of interpersonal violence, including rape, battery, and child physical and sexual abuse.

… [Studies] that use long follow-up periods tend to show alarming rates of sexual reoffending among rapists. For example, Prentky, Lee, Knight, and Cerce (1997) reported a 39% sexual reoffending rate over a 25 year follow-up among rapists who had undergone sex offender treatment. …

… In addition to high rates of reoffending, several studies have shown that among incarcerated rapists the actual number of sexual crime committed far exceeds the number of adjudicated charges against these men {my emphasis}. For example, Abel and colleagues (1987) reported that when given assurances of confidentiality, 126 identified rapists admitted to 907 paraphilic acts against 882 victims. Weinrott and Saylor (1991) conducted a similar study of sex offenders in a state treatment program. The 37 rapists in the study had been charged with 66 offenses against a mean of 1.8 victims. Yet under conditions of confidential self report, these same 37 men admitted to 433 rapes against a mean of 11.7 victims.

The varying types of reactions survivors might have don’t make this any less of a violation of someone’s bodily autonomy.