With a TA of 340 I'd probably try and lower it, and I'm not a fan of lowering it unless needed.
epxroot said:The low CYA level + kids swimming + sun would be why I lose so much FC during 9 hours, correct?
That could definitely do it.
Should I raise my FC now and then lower my pH later on tonight?
Yep, add chlorine and wait about an hour, with pump running, and then adjust pH.