Just got back from 4 days in a universal hotel and 2 weeks in a villa in kissimee. If you stay onsite as others say you can get in early early and then back to your hotel in the afternoon. We ended up doing full days at each park and only went to one park (epcot) twice because we didn't "finish" it. I personally would not go back to magic kingdom or epcot now but I liked typhoon lagoon, animal kingdom and hollywood studios.
Universal is better. Your 7yo dc is a bit young to get the most out of Universal as there are some thrill rides that you have to be tall for but you 9yo will be ok to do everything. The harry potter stuff is amazing, not to be missed, and my 8yo loved it. Definitely stay on site at the loewes or I think it's portofino that included early park entry and fast passes with the room. That way you can get in and have a chance of getting on hagrids motorbike before the queue hits 2 hours (at 8.30am!!).
Disney is ridiculously expensive and the exchange rate is not in our favour. Even supermarkets were very expensive. Its nice that the rides aren't overwhelming.
You Need genie plus to get lightning Lane access to some rides. You have to be on it at 7am booking onto lightning lanes. I paid for a couple of rides because I'm not willing to queue for hours. Flight of passage in animal kingdom is great. The seven dwarves mine train at magic kingdom not worth the extra cost.
I didn't do loads of research before my holiday but the day/night before I did loads of reading of online blogs to get my strategy for the next day. The idea is to queue less and not tire yourself out criss crossing the parks.
Typhoon lagoon is great for rest days but you do have to get in early to reserve a spot on sunbeds. We did a full park day then a rest day in the villa. By the end my dc wanted to go back to universal but I couldn't face it and fortunately our 2 week pass expired.
We got our tickets on attraction tickets direct. The tickets cost the most!
You need restaurant reservations which have to be sorted months in advance. We did get late availability e.g. booked a character dining experience in animal kingdom and one of my family got into her favourite restaurant in epcot by late availability. You have to be organised and go early on the day to get a booking or the day before as people cancel.
I didn't get to go to space 220 at epcot which would have been fab but very expensive.
If you aren't on a budget it will be great. Otherwise it must be hard to do cheaper. Even £10k doesn't go far now!