Good spots are along the front of the viewing area facing Magic Kingdom along the concrete wall. Children and short adults cannot see the castle either unless they are over 5 feet. Further back unless you are over 6 feet tall you cannot see the castle. I would be at the viewing area at least 15 minutes before the fireworks scheduled time. Actually I would go up 45 minutes before, get a drink and keep an eye on the viewing area to get a spot. During the week it usually isn't that crowded except during the
DVC busy period, October through the Disney Marathon weekend in January and holidays where WDW is crowded.
BTW, the kids would be able to see the view of the fireworks better from the north elevator lobbies on any floor 8 and above. There is no music though. Some watch it on the bridge but they do restrict the number of people on the bridge if it gets too crowded. And it can be very windy on the bridge. Others watch it from the parking lot.
Have you considered getting a theme park view 2BR?