I think if you're not dedicating enough time to a proper Universal parks trip it wouldn't be worth it, especially July this year. Why? Parks will likely be busier than normal with the new HP coaster opening (if you're not planning on getting express pass this would probably kill your enthusiasm for Uni altogether).
Secondly, I think 1 day is just simply not enough time to see and explore WWoHP at both parks, especially if your kids are into HP! Our recent 4 day trip was spent at least 50% of the time in those park areas, and that's probably because we left and enjoyed other parts when HP got really busy. So, if you're plan is to ONLY see HP worlds I suppose with a park-to-park it is possible but then you'd miss out on the rest of the parks. Again, keeping in mind July will be a busy time of the year. I'd hate to plan only one day and miss out on parts of HP let alone other parts of both parks.
I would also check height requirements for the rides as has been mentioned. This would have been a deal breaker for our last trip (luckily my 7yr old is a big kid and could ride everything except Hulk). If he was too short for Forbidden Journey (the Hogwarts ride in Hosmeade/IoA) that would have made me postpone (or maybe do Disney).
If your kids are definitely into HP it's worth it to spend a few days dedicated to Uni. You can stay there, benefit from the resort ammenities/bonuses, get discounts on longer stays, and explore all the nooks of these great parks as well!
