Are these veranda rooms? On the Magic, there is no Deluxe Family Oceanview Stateroom without a veranda.
We have cruised on the Magic in both Deluxe Family Oceanview with Veranda (Category 4) and Deluxe Oceanview with Veranda (Category 5/6). The main difference is that the Family rooms have a drop-down bed next to the veranda and are thus a few feet deeper to accomodate it. On our last cruise, my son slept in that bed, with my daughter on the top bunk. That allowed us to use the sofa for seating even in the morning or evening when the beds were set up. The downside was that it was difficult to get to the veranda when the drop-down bed was set up.
The room had a cabinet opposite the drop-down bed that was very useful for clothing storage on our 8-night cruise. I don't remember a similar cabinet in the Category 5/6 rooms, but I could be misremembering.
All of the Category 4 Family rooms are on Deck 8. We have stayed in both Deck 8 Fore and Deck 8 Aft, and both locations have advantages. The Fore is right below the adult-only areas on Deck 9, which made it easy for my husband or me to dash to Cove Coffee for a quick drink or snack. The Aft is below Cabanas, which made it easy for the kids to visit the Beverage Station. With Fore, it's quicker to get to the activities in the Walt Disney Theater and the adult lounges. With Aft, it's quicker to get to meals in Animator's Palate and Rapunzel's Royal Table. We did find that both the Fore and Aft elevators were usually quicker to catch than the Midship elevators, and the stairs were less crowded.
I wouldn't choose the Family room based solely on the extra square footage with relatively young children, but if having the drop-down bed (or the space it would occupy) would work better for you, it may be worth the extra money. We started going to Family rooms once both kids were teenagers, and both they and their clothes and backpacks took up more space.