Going by the title of the thread, I assume that you are using a digital pH-meter. As far as I know, the reading on these is unaffected by high FC. I don't know this specific meter. so can't judge its accuracy in general. I am using am Aperga pH60, which works well as long as I keep it properly calibrated, and that seems to work fine at higher FC levels, where colour based chemical pH tests fail.
But generally I'd be careful with adjusting pH while slamming. Adding chlorine to get to shock level will increase your pH initially (which seems to be what your oldest kid is complaining about). But the pH will fall again once that chlorine gets used up. So, by bringing that high pH down with muriatic acid while still on high FC might lead to a very low pH after the slam. Take chem geek's pool equations spreadsheet and play around with the effects on pH by adding/creating chlorine and also by the various ways of chlorine being used up. Usually, the creation (by a SWG) or addition of chlorine and the following decay of chlorine is a pH-neutral cycle. The general trend of pH rising over time that needs to be corrected with muriatic acid, is due to CO2 out-gassing.
During a very long slam where you add chlorine multiple times you could measure the pH (with a chlorine level independent pH meter) always at the same FC level. The change in pH at that same FC level is then obviously the result of CO2 out-gassing, that can be corrected with muriatic acid. But I wouldn't correct any more than that. And there's probably not much to correct anyway, because at high pH, CO2 out-gassing will slow down (add a drop of lemon juice to carbonated water to see the effect of pH on CO2 out-gassing...).
Hope I got that right from the multiple threads on pH in the deep end...
