If it's about the kids, then I really disagree with the advice to choose whichever itinerary you prefer. At their age, it does not matter what ports you visit. So, considering their enjoyment:
My vote is for the Magic. The kids clubs will be less crowded, the pools will be less crowded when you take them there, and the ship itself is less overwhelming to a small person. The dining rooms, for example, will not be as filled with people. For family time, the lounge right off the atrium is wonderful for families, they often have music or games there, and it isn't closed in at all (it is open to the walkway going past making it feel very welcome). I understand the Frozen Show is updated and better on the Magic, something the kids will love. And the theater seating area isn't as cavernous on the Magic, everyone (especially little ones) has a chance to get closer to the action.
Castaway Cay will be much less crowded with the Magic than with the Fantasy.
The benefit to you is you will have a balcony if you choose the Magic, and that's pretty cool; I really like the adult clubs on the Magic; as long as you aren't going to Nassau (I assume neither option goes there) the ports Disney visits are all pretty nice (we all have our favorites, but most of them have really cool excursions and ship dock areas); the lunch buffets are less hectic; lines at GS will be shorter,
I agree with everyone that either one will be lots of fun. I just offer considering something other than the itinerary, as that seems down the totem pole in importance for this cruise.