@PUNKEY,
Masturbation in children is NOT a sign of sexual abuse or exposure to porn or other inappropriate material generally speaking.
Unusual masturbation could indicate a need for concern, however, generally criteria for becoming concerned include:
Child masturbating to the exclusion of other activities.
Child persisting in masturbating in public when parents have spent some time assisting child to understand where it is appropriate and not appropriate. (Often kids will unconsciously touch themselves when watching TV and other zone out activities even then, and need gentle reminders)
A child of this age penetrating vagina or use of objects
Child expressing that they cannot stop masturbating and think about it all the time
Child using very adult language when masturbating.
Child appearing upset or angry when masturbating, child of this age appearing very ashamed of the activity or being angry or very emotional when re-directed.
Child harming themselves with the activity
Even in these cases it is quite possible that there are other emotional reasons for the concerning level of masturbation, and that there has been no abuse. One should never ASSUME abuse in such situations.