Our kids were 8 and 10 on our first trip, and they had a blast. Both were (and still are) big HP fans. No matter how many times we go or how long, they always have a blast and immediately ask when the next trip is. As they've gotten older (13 and 15), they've grown to love other areas like the Simpsons, Spiderman, and Transformers.
Doing both Universal and Disney in the same trip will be expensive, there just isn't any way to avoid that. The first two days are the worst, whereas the cost/day dramatically drops for longer stays (or multiple trips with an AP).
I usually recommend 3 days minimum, staying onsite to really experience UO (at a deluxe resort for the EPs if possible). One day dedicated mostly to each park, and a 3rd to hit favorites. But DW has so much, that cutting it down to 4 days makes you miss a lot there. So in this case, 1-2 days might be better. If cost is not a factor, a single day doing a VIP tour would work. A 2-day trip staying at a deluxe resort for one night would also be a good option, if you are willing to give a second day to UO...