Firstly, sadly the UK free dining for Disney next year has closed. It opens up again at the end of April 2019 until November 2019 for stays in 2020. It is standard every year.
We have done a split stay with Disney/Universal twice (including August 2018) but we prefer Loews Royal Pacific over Hard Rock which had a more lively teenage vibe. Royal Pacific felt more chilled and laid back. There isn't a huge walking distance involved and you can always get the water bus back (free) or use a rikshaw (small charge.)
We did Universal first then moved over to Disney hotel using Uber for the airport to Universal and then to Disney. Got the Magical Express back to the airport from the Disney hotel.
If you haven't already discovered TheDibb website then have a look. It is full of Disney obsessed Brits who have been many times and can advise on every tiny detail of Universal and Disney.
I agree with PP with children aged 9 and 6 Disney probably has a lot more to offer than Universal. The first time we went the children were 11 and 8. Although Volcano Bay is incredible so if you think your children will like the water park it might be worth 3 nights there. The FOLP (front of the line pass) that comes with your hotel stay makes it much easier to go to the parks when you like.
Disney wise, their buses are fantastic. We usually go to Florida for 2-3 weeks. We have never hired a car. We aren't shoppers/mall people, we are there for the parks. We did use Uber to get to some places and it worked really well.
We are back, again in 2020 as it is a GCSE year for Ds1 2019.
Don't assume it will be a once in a lifetime trip, I said that. Ate my words. 