I'm leaning towards single-day tickets for your family because the process of park hopping (even between MK & Epcot via monorail) at WDW will likely take up one or two extra hours of your day (unless you'd already planned a mid-day break at your resort). WDW is huge, and the free transportation, while convenient, is slow. When we're at the World, we find ourselves thinking in "Disney time", in which it feels normal for a 5-mile road trip to take up to an hour.
Also keep in mind that the parks are large and each has a unique style and good places to eat. With the possible exception of Hollywood Studios, you'll likely find that your days are quite full exploring each park, so you may have no urge to hop. It can be great to settle into the mood of just one particular park each day- enjoy its rides, decor, shows & restaurants, then focus on another park the next day, rather than rushing from one park to another, not taking the full experience in.
You do have a point about the magic hours- if that's the deciding factor I'd check park hours for each park for each day you plan to visit, & ask yourself if, honestly, your family will actually use them (because they're always late-night or early-morning- don't forget your kids will need to eat something for breakfast first, then transport to the park all before the early-morning hour begins). I like evening magic hours but that's because my son is 10 & very flexible with his sleep schedule- many children are not.
If you don't think your family will be able to make much use of EMH, I recommend sticking to single-day tickets. You can always upgrade them to hoppers if you change your mind at the parks. You can only get FastPasses for one park per day in advance, anyway, so why pay extra upfront for something you're not sure about?
(Sorry for the large font- it's a computer glitch.)