For one, flight delays are always possible. If said trip is in summer months- afternoon/evening thunderstorms are often a problem. In winter, a blizzard any place in the lower 48 can muck a flight.
If said family has never been to MCO, they also have to get from the gate to the Disney ME area. First there's the walk from the gate, then the MCO monorail, then finding the bus area. Sounds simple but the distance is quite a bit, and the bus is on lower level of a large airport. Also add time if they plan to get their checked luggage.
Or- if they use ME tags, we'll need to assume they don't want to wait for checked luggage to arrive at the room.
If they do online check-in they can bypass front desk, but they might want the welcome packet that has the hotel map, and hotel information. Myself I always like the resort activities list. Also, if they have
DDP, they might appreciate an explanation of how that works.
They might be the first stop, or they could be the third stop.
So if all goes VERY well - they MIGHT be at their hotel an hour after landing. If just the ME bus timing is slow + they are the third stop= it may take a full THREE hours form landing to hotel arrival.
So I wouldn't PLAN on arriving in time for fireworks, but it MIGHT happen.
If you are going for 5+ days, then adding a day to their tickets isn't that much $, but don't buy the extra day in advance. modify the tickets MID trip. You can't add extra days to tickets once they are fully used any more, so they must do it BEOFRE the last day.