Unfortunately, families cannot access their students' rooms without the student letting them into the building/being present while they are there. While I suspect that is not what you wanted to hear (and I know that 99% of families would never do anything untoward), those rules are in place to ensure the safety and security of the student residents.
So you will need to meet your student at a time when they have a break in their Pre-Orientation activities to finish bringing things into their residence hall.