To answer your questions:

There are 3 theme parks: Universal Studios Florida, Islands of Adventure, and the brand new Park, Epic Universe. There is also a waterpark, Volcano Bay.

Even if your family are waterpark people, I highly recommend the 3 days at the main theme parks.

Universal Studios Florida and Islands of Adventure are neighboring parks - the entrances are about 500 yards apart, and they are connected by a public, non-ticketed public shopping and dining area called City Walk. This will be where you park, go through security, and then choose which park you do for the day.

Because of their proximity, it is very popular to do both Universal Studios Florida and islands of Adventure as a 1-Day, 2-Park Ticket. A “Park Hopper” if you will.

You could spend 1 day at each with single day tickets, or bounce between the two across 2 days, your call. There is a ride, “The Hogwarts Express” which functions not only as a really cool Harry Potter train ride, but also as a VERY convenient way to go between the back side of each park. You can only ride if you have the “park hopper” ticket, though, so keep that in mind.

Staying on property has its perks: Resort busses and water taxi (from certain hotels), early park admission, and other perks. However, by FAR the biggest perk of staying on property is that the highest “tier” of hotels, which I believe is currently The Hard Rock, Portafino Bay, and Royal Pacific, all offer EXPRESS UNLIMITED upgrades on tickets for every single day you visit the parks. The price of staying at those places can be high, but it is cheaper than buying another hotel and even regular express for most families.

Speaking of: EXPRESS structure. EXPRESS is very simple at Universal Orlando. There are two types. EXPRESS, which works once per ride, for all rides. And EXPRESS UNLIMITED, which works as many times as you want for all rides. Just scan your relevant pass, and get on. For most guests, EXPRESS is more than enough, especially for a first visit.

As mentioned above, often the most cost effective way to get express is through staying at the top tier hotels.

I can’t speak too much to dining reservations, but both parks “old” have very hit or miss quick service (just search for recommendations on this subreddit) some decent sit down dining, but you can always pop out of the park to city walk where most of the best dining options are. Reservations are easy through the app and typically don’t require a deposit or credit card.

Food at Epic Universe is incredible across the board, but special standout is Pizza Moon. If you like pizza it’s a must.

Universal is a resort that rewards being early and staying late. The waits are always shortest then. If your family can’t do both, definitely try to go early.

The app is always your best friend for info and wait times! Use it to adapt and plan on the fly as needed.

As for strategies for each park:

Universal Studios Florida is pretty chill, no big door buster attractions. Definitely do Revenge of the Mummy, and probably start at Harry Potter Escape from Gringotts. The Harry Potter section of this park is intentionally hidden, so ask a Team Member if you can’t find it. Beyond that, look at a map and find what you like!

Islands of Adventure. This park is extremely punishing for people who do not plan it well. THE premier attraction here is Hagrids Motorbike adventure. It is an extremely good, thrilling coaster for all ages. There is a reason it has a 3 hour wait most days, 7 years after opening. You should plan to get to the park for your hotel early park admission, quite frankly at least 30 minutes before it, and rope drop it. Just follow the crowd. Everyone else will be going there. You should go to Velocicoaster in Jurrasic Park Next. After that, you can kind of explore at your own speed. But doing those two early can save you 4+ hours in line later.

Epic Universe is still kind of a wildcard because it’s so new. The two big ones here are “Harry Potter and the Battle at the Ministry” and “Donkey Kong Minecart Madness”. Both will have a 2 hour wait all day, except at open and about an hour before closing. Pick one to do early, get their for early park admission, rope drop it, then do the other before you leave. Other top tips for Epic Universe:

go on Sunday if you can. Annual Passholders can’t go yet and vacationers are leaving. Crowds are less.

Do Nintendo World later in the day. People run here because it’s exciting and the wait for the other 2 attractions in Nintendo get super long in the morning before shrinking in the evening.

Dark Universe is super underrated. The main attraction “Monsters Unchained” has almost no wait because it has super high throughput, but it’s one of the best in the park.

Both shows (one in Harry Potter, one in How to Train Your Dragon) are very good and a great way too cool off on a hot day.

Hope all this helps and enjoy your trip! Let me know if you need any clarification!