What can I tell the parents the main difference is?
Well, on your end, if you create a classroom, you will have the ability to see student progress and also collect their answers to the short answer questions. You would also be able to assign the course or activities you want them to do.
If it’s just for fun, you could just tell them to use it (and I might suggest http://code.org/learn as a good jumping point as there are a ton of open ended hour of code style activities, many of which don’t require a login, but if you need any kind of accountability or want to see what they do and/or keep them all doing the same activities, I would suggest the classroom.
Hope this helps!