Change permission in file

My nephew tablet get infected with malware, following some suggestions from thsi sub, I rooted the tablet and uninstall some malware apps. but there two malware apps (engriks and adobe air, but it's not he app from adobe) can't be uninsalled (right now I disabled the apps) I find the apk of the malware apps in '/system/private_app/' I try uninstalling the apps with Titaniun Backup and the apps and the apps still in the system.

How can Uninstall or delte the apk of this malware.

Is a cheap chinese tablet NeuTab K1.

Thansk in advance