Swimming with pH of 8.0 won't be dangerous, but might be uncomfortable to eyes. You can get muriatic acid at Lowe's or Home Depot. It is fine to swim a few hours after adding it. Just want to make sure it is mixed in well. Be careful while adding it --- gloves, glasses, upwind of fumes, pour it close to water surface to avoiding splashing. Nasty stuff.
Also, take a look at this guide about lowering alk.
http://www.poolsolutions.com/gd/lowe...p-by-step.html
Have fun with the grandkids!
EDIT -- Actually, after reading something Ben wrote about high pH, I don't even think it will bother their eyes at 8.0. But, you'll still want to work on lowering it a bit.
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