Since it was rooted, theres a good chance that it had a custom recovery installed; un-rooting it via Superuser does not delete your recovery.
Reboot into recovery to check (I think it's power+vol up+home?) what type you have. If it's TWRP, CWM or Philz, there's your problem.
Just flash stock recovery (should be a link on xda somewhere) and you should be OK and *fully* unrooted & able to get OTA's. I'm pretty sure you can still flash a recovery IMG while unrooted if you do it via ADB or Odin, just make sure ADB debugging is turned on in your settings.
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Thanks! Man. I did as u advised and have unrooted my kids tab now.