Hi and welcome back to the forums!

The plan looks reasonably set out and I presume you have discussed it with them and checked they are happy with the pace of the trip? One area I would look at, is the time they actually have to check out Bryce canyon and Zion NP, it looks a bit rushed to me, especially with a side trip to Horseshoe bend. I would consider dropping Montezuma castle and head into the Grand canyon that day and exit via Desert View drive the following day and head to Bryce canyon, giving them chance to explore more of the Canyon the following morning before heading to Zion for the night, and stay another night to give a full day to explore this amazing park. Personally, I would not use St George as a base for the parks, there are lodgings in Bryce Canyon city (including Ruby's Inn close to the entrance) and for Zion NP, Springdale is a great little town close the entrance and with a Shuttle bus service into the park. From Las Vegas, they could drive through Death valley and stay in Lone Pine, Ridgecrest, or even Lake Isabella area to give a little more time to drive through Sequoia NP.