Question regarding twilight check in

We, family of 4 are flying Lgw-las next month and have a hotel booked the night before at gatwick. We're flying economy so I've paid extra for seat plus to give the little ones some extra room but having done so it means we can't check in on line and have to do this at the airport. My question is can we do the twilight check in the night before to hopefully get the seats we want but not drop the bags? It would be handier to have these at the hotel with us and just drop them the next morning. Is this possible or do we have to drop the bags at the time of check in?