We go to Universal quite often. If you want to do the room, you could, but I don't think you need it. Would it save you some times in line? Yes, but if you are focused on seeing HP, then the amount of time saved for one day probably isn't worthwhile. If you were going for two days and everyone would be going on all the rides ,then it would be more worth doing. We've done US both with and without EP.
Ug! you don't need an elaborate touring plan at Universal, it isn't Disney, especially fi you do decide to get the Express Pass and arrive early.
If you DO get the room....I know you said you'd only go to the Universal parks one day, but you could potentially use the hotel pool the next day (check out day), or at least the family members that are listed on the hotel. Depending on the condo location, that might be a fun activity for the older kids - maybe while the two little ones nap?
another plus of the throw away room- in August- is that you could most certainly use it during the day - again- if the little ones need a nap, or everyone wants a break from the heat.
If you DO get the hotel, you need to visit the hotel to check-in and get the Express passes. That shouldn't take long, but if you are stopping by the hotel, you might as well park there. The one night parking fee would allow you to park there both on arrival day and departure day.
I suggest maybe thinking through which rides you actually plan to ride. If you are going to do SR, then you don't need express pass for the SR lines. Early in the AM, the EP and standby lines are almost the same. Late at night, the park also quiets down a bit.
For the little ones, there is actually quite a bit they can do: Barney, Animal actors show, Seussland, Curious George play area, Minion Mayhem (non-moving seats if too short), ET (if over 34' w/adult), Hippogriff (over 36' w/adult), Fievel's playland (In the area is a ballroom that's fun for everyone, and a water play area that would be fun for everyone), Kang and Kodo's (Dumbo but with alien theme), Pteradon Flyers (read out whoca n ride...mostly kids basically need kid+adutlt to ride), Olivaner's, Shrek, Blues brothers show, Sinbad show (expect to get wetif in splash zone), the Dino exploration building, character greets, and the night time fireworks show.
Everyone can explore both HP lands, including the HP shows. Everyone can explore all the gift shops. Everyone can explore Citywalk (at least the non-bars) and use the frequent hotel shuttle boats.
The Harry Potter interactive wands at about $50 each, but with them the kids can go around both lands doing all the spells. they come with a map that shows all the locations If I was in your shoes, and you think you will go back to Universal- and I was deciding how to spend my $, I'd choose wands over paying for the throwaway room. There's no Express pass for doing spells; everyone has equal access. Mostly though, wands are best for kids that are around age 8/9 and up. To do the spells, you have to be able to hold the tip of the wand straight. Smaller kids don't quite get how to do it, and then they can get frustrated. The staff helps at many of the locations, and the younger kids will enjoy watching the spells almost as much. The younger kids could even TRY using their big sister's wand (without buying their own). Universal makes it sound like you need your own wand, but really you can share. the wands are fun for adults, too. If you need to explain WHY the little kids are too little...just tell them Hogwart's students get their first owl post invitation on their 11th birthday!
Oh, if you think you'll do the wet rides- they are VERY wet. Dress accordingly, or wear a suit under your clothes. Again, access to an onsite room might actually be useful. Again, especially if anyone wants to take an afternoon nap in the AC, or escape an afternoon downpour. Universal isn't Disney. Royal Pacific is only a short walk (10min from park gate) or boat ride from the parks, and boats run more frequently than they do at WDW. Even if you don't get the room, you can opt to visit one of the hotels for access to the lobbies/AC/quiet. it will almost certainly be much closer/faster than going all the way back to your condo.