IF they're willing to get to the parks at least 30min before early entry begins, they should hit a decent wait for Hagrids. Earlier is better, although then you're obviously waiting longer for the parks to open... Anyways, if they're through the big archway and can see the entry turnstiles, they're in great shape for Hagrids with a 5-10min wait. Then proceed to enjoy Hogsmeade at their leisure. This is also a good time for Forbidden Journey, although with Express Pass that won't matter too much when they do it - EP isn't active during early entry.
They can do whatever they want to do in Islands, take the train to Studios, and then start in on Studios.
Staying onsite at a boat resort makes it really easy to head back to the room for a break mid-day. If they're doing early entry and intend to stay until 9pm, a solid 2-3 hour rest in the middle of the day might be a good option. Ideally, this could be timed after heading into Studios and making their way to the front of the park - sticking along the right side of the park would take them past Fast & Furious (though I'd recommend skipping this), Mummy, Jimmy Fallon (also recommend skipping on a short trip), Transformers, Minions, and Rip Ride Rocket.
After a hotel break, coming back the opposite side of Studios would be going past the Bourne Stuntacular, ET, Simpsons, MiB, and then back to Diagon Alley to explore in the evening, have an ice cream / Tales of Beedle the Bard break, ride Gringotts, etc. Of course, this could be done without a break too, picking a longer resting meal for lunch & dinner.
And depending on time, then take the Hogwarts Express back to Islands to end the night in quiet Hogsmeade - another ride on Hagrids (at night!) or spells after parks close, when everything is lit up all pretty with minimal crowds.
If they're already doing an onsite hotel, and especially if they switch to an Express Pass hotel, it's probably worthwhile to upgrade to the 2 day ticket at that point (since the hotel EP is good for arrival and departure day). On the other hand, with EP they can get an awful lot done in one long day.