When my family went to Orland last summer in 2017 my girls were about the same age (8 and 6) as your girls will be in 2019. Just a little background, we've been going to Orlando every year, 2-3 times a year since 2015. We normally spend four to six weeks there in the summer and two weeks at other times. Since we go for so long in the summer, we pretty much visit all theme parks (Disney, Universal, Seaworld, Discovery Cove, Busch Gardens, Legoland, and water parks). We normally stay on-site, depending on the park we're going at the time. Since Seaworld and their sister parks don't have their own hotel, we rent a vacation home for that portion of the trip. Needless to say, we (okay, I) LOVE theme parks!! Our favorite park is the one that we are at at the time of questioning!! To answer your question:
1. My personal favorite is Portofino Bay. But I generally book Hard Rock most often because of its proximity to the parks (and The Palm). Royal Pacific is really close to the parks too but just a smidge (I'm talking a minute or two here) further than HRH. But the one or two minute difference counts when walking with 6 and 8 year olds.
2. I don't think 8-10 days is too long at Universal, especially with visits to MK, SW, and other places you mentioned. We normally spend 8-10 days at Universal and we don't tend to leave the property once we are there. But that all depends on how you tour. At UO, our touring is very relaxed. No rope drop most of the days. Spend several hours at the parks. Come back to the hotel to rest and freshen up. Have dinner and go back to the parks at night if we feel like it. We also love the pools. My girls can spend all day at the pool. They also love the water play area in both parks (Seuss Landing in IoA and Curious George in US). They spend a few hours in each.
3. I never book with a
travel agent. I find it to be more work than doing it myself because you have to go through an agent everytime you need to change something. But a lot of people I know use an agent when they plan a Disney World trip. A Universal trip is not nearly as complicated as that if you have Express Pass. No need to make dining reservations or Fastpass.
4. I think your girls are an awesome age to go to Universal!! I don't think all the rides are for older kids. For some reason, Universal has that reputation but I absolutely disagree! The only rides my 8-year-old can't ride (height requirement) are Hulk and Doctor Doom. My 6-year-old can't ride Rip Ride Rocket in addition to what her older sister can't. That's only three rides in all of Universal Orlando! Granted, the ability to ride doesn't equate the willingness to ride. To this day, I can't get them to ride Mummy (They said the queue was too scary!)
The first time we went to Seaworld we spent a day and a half there and we didn't even do their three big coasters. There is so much to do there. The first day we were there from 9am to 10pm and we went back the next day from 12 to 6pm.
I hope this helps!!