How to Unroot " Galaxy tab 2 p3113 8gb wifi " My 9 year old son rooted his tab on his own, against my wishes. I tried a factory restore and used Kies to load via PC although this did not restore the tab.

It's still showing as rooted. Can anyone help as it seems he should have installed software prior to rooting from what I have read. Is there anything I can do to restore it to factory-defaults? Please anyone who can give suggestion I would greatly appreciate it.
Just reinstalling factory ROM will remove root access. I am assuming he has a custom recovery image installed? Also, how exactly is it showing that its rooted? Is a superuser app installed? Stock ROM?