Personally with that list I would skip the water park as it would be tight to get to everything with the limitation without being rushed. If you had to get it in I'd allot either the 3/4 da or 1/2 day to it. I haven't been so not sure how long it would take, but if the express pass gets you through it quick i would do the 1/2 day there as it probably has less to do overall, but depends on how long you think you could spend doing the same slides or lazy river/wave pools.
Since your kids are Harry Potter fans I'd probably dot he first day at Universal Orlando. Work your way back to Diagon Alley, hitting any rides on the way you want to do. Then go through everything you want there since it would be a new area and it has a lot to explore for fans. Then take the train to Hogsmeade and do Harry Potter stuff there, then ride it back (it's different each way). After this finish up Universal Studios and reride anything you want if you have time.
I'd use the 3/4 or 1/2 day then to do IoA focusing on the non Harry Potter stuff since it got covered in the full day and just hitting what you really want out of it when done.