They, like all Disney resort guests, will be considered Disney resort guests from around 6 AM on check-in day until the last park closes on check-out day. Doing online checkin would allow them to go straight to the park rather than stopping at the resort first. But if they have luggage to leave with Bell Services or if they need Key to the World cards to use as their park tickets (as well as their room keys, charge to the room, etc.) then they do need to stop at the resort before traveling to the park.