Tablet repair probably isn't something you'd get a specific certification for, unless you want to work at a contracted repair shop that specifically promotes iPad repairs, or Galaxy Tab repairs, or Kindle repairs... etc etc etc.
Essentially, you should be focusing on CE repairs if possible. The basic repair skills are going to be similar across many small CE devices that have an integrated/unified circuits approach - ie everything is either directly soldered into the main PCB, or there are a haphazard mix of soldered and snap-in components (likely unique hardware fasteners and unfriendly electrical/data connections and pin-outs)... and many of those devices have warranty-considerations, since stickers and whatnot will reveal unauthorized work - so you'd likely have the knowledge to work on many devices, but you'll be limited to what manufacturers have authorized in-warranty repair services at the shop you work at... if any have even provided such certification to that shop (it could be limited to out of warranty repair, like quite a few general "We Repair All Electronics!" type shops).