My dd spent last year at Oxford, dh had a business trip in the UK that coincided with the end of Trinity term so we scooped up dd and best friend and spent a week in Wales. As we were leaving, we saw lots of families with younger siblings in tow packing up cars with dorm stuff. I realize it's not the same as drop off, but just wanted you to know many folk appeared to solve "what to do with younger siblings" issue with bringing them along.
That said, I would defer to C & H about what they want to do. If Calvin wants Hobbes to come, and Hobbes wants to be there, I would bring him. Think of it as an early college visit :D Also I would assure Calvin that he will see lots of other families in the same situation.