If your daughters are huge HP fans - I think one day is worth it.
1) Be there before opening. Do the mad rush to WWOHP. Get in line for the Wand Shop. See the show - then spend LOTS of time deciding if you want to buy anything at D&B. I recommend bringing a pen - and buying postcards and Hogwart's stamps. Have the girls mail themselves a postcard at the Owl Post with the Hogsmeade postal code (have them fill out the postcards at lunch).
2) Go to Honeyduke's and Zonko's. Browse well, as the later in the morning it gets, the more crowded. Don't buy chocolate frogs now. You can buy those at the front of the park in the main store. Spend time browsing, buy now if you want, as later in the day, there will be a line to get in the store.
3) Have lunch at Three Broomsticks. Plan this about 11:15-11:30 - or you will wait in line to get in. Order your Butterbeer in the mug now - so you don't have to wait in the Butterbeer line.
4) Go on the CASTLE TOUR only of the FB ride. It is NOT obvious how to do this - but if you keep asking staff - they will get you to the right spot. Be sure to spend the time to watch the little shows that happen during the line. There will NOT be a line for this.
5) Try to catch one of the little shows that happen outside. You can get your picture taken with the cast afterwards.
6) Get your picture with the conductor by the train.
7) Be sure to take the time to LOOK in all the store fronts. For example, every so often - the music pages fly in the windows of the music store. The detail is amazing - and it takes time to appreciate it all. Try pumpkin juice.
8) Once everyone agrees you have seen WWOHP - then leave. I suspect your girls will like Seuss Land - and there are a couple rides they can go on there. And check the ride heights/shows in the rest of the park to see what might interest them. IOA is VERY nicely themed - it is quite attractive to walk around and see stuff. Don't be surprised if your girls want "Thing" T-shirts by the time you leave.
9) You can buy wands, choc. frogs, and the HP plush pets at the main store at the front of the park. Personally, I think it would be more fun to buy the wand in WWOHP. You can't buy everything at this store though - so I would buy the must have "thing" in WWOHP - even though it's so tight to move around in the stores there. Just be patient.
For true fans - even without doing any rides - I think you can spend 4-5 hours in Hogsmeade. It can be overwhelming - and you can be tempted to leave out of frustration from the size of the crowd. Don't rush out because of that. Sit down, take a break, and enjoy the view. Hogsmeade is brilliantly done - the attention to details is great. But you have to take the time to stop and LOOK at it. Maybe even do the castle tour twice.
And the rest of the park is nice, too. You can get your money's worth out of a one day ticket!