My youngest is almost 4, and I will not bring her to Universal for several years. The biggest drawback to UO is the kiddie rides are mostly contained in one small section of each park (one in Universal Studios and the other in Islands of Adventure), and there are many true thrill rides that even my 3rd grader chickened out of, even though he met the height requirements. I thought Seuss Landing was adorable, but it's not a full day's worth of amusement for younger kids. AND because the kiddie rides are all clustered together, the family will have to split up a lot for the older ones to ride the bigger attractions. There are a few things here and there for the whole family to do like the Minion ride/simulation, but on the whole, the parks are not set up to be conducive to families with older and younger kids. I just got back from Legoland as a solo parent with a 3-year-old, a 9-year-old, and an 11-year-old, and it struck me as so well thought-out that by nearly every height restricted ride my older kids wanted to go on, there was a kiddie ride or play area for my youngest.
I honestly think you'll regret skipping out on Disney to do Universal at this point. Your younger kids won't get much out of UO. Honestly, so many of the rides are motion simulators that YOU probably won't get as much out of it as you'd like.
I mean this in the nicest way, but it sounds like the desire to go to UO is really driven by your love of HP, not the interests of the rest of the family. I don't say this in a snarky way at all because you're a person, too, and you are also entitled to want to experience certain things. I shamelessly take my kids to theme parks because I enjoy them (and DH stays at home because he does not). In your case, at this point, it doesn't sound like Universal would be nearly as enjoyable for your family as Disney.
If you really want to go to Universal this trip, have you considered splitting up and buying tickets to Universal ONLY for you and your oldest (who has read the series)? Yes, it's ridiculously expensive for one-day tickets, and you'd need the park-to-park tickets (because half the HP is in one park and the other half is the other park, and the only way to ride the Hogwarts Express - different story in each direction - is to have the hoppers). However, it's much cheaper to do it that way than to buy for 7. And if your DH has only 'enjoyed the movies,' he will probably LIKE the HP world (because it's cool), but it doesn't sound like it will be a huge sacrifice to miss it this trip. You and your 4th grader can easily do all of HP in one day without express passes. Since you don't do motion simulators, that knocks out a lot of the other rides in the park, anyway. Factor in the cost to buy a wand (and do the wand ceremony experience, if possible) because that $50 is SOOOO worth it to make the windows come alive. Could DH take the other kids to Disney himself or do a resort day with them?
Otherwise, you're better off doing all Disney now, and then doing an extended Universal trip in a few years when you return to Orlando with older kids who will better appreciate (and be tall enough to go on) most of the rides. My oldest are now 9 and 11, and while they like Disney, they're at an age where they're more into Universal's offerings, and would be quite happy to only do an after 5 ticket or party to get a quick Disney fix while on a Universal vacation.
