Hey there Richard,
Thanks so much for your question and welcome to planDisney!
Star Wars: Galaxy's Edge is such a fun place to visit - especially with a little Jedi! You've definitely come to the right place because I faced this same question when I took my son at 5 years old. I went back and forth between whether to build a droid or lightsaber with him.
Ultimately, I ended up deciding to visit the
Droid Depot with him because, knowing my son, if he had a lightsaber then he was going to hit things with it - that's what they're for, right?! He had a remote control car or two prior to us building his droid so I felt a bit more confident that a droid might last longer after we returned home than a lightsaber. To that point, we still have his Chopper (C1-10P) droid to this day!
I think it really comes down to your grandson's likes, dislikes, and temperament. The lightsabers built at
Savi's Workshop are extremely high quality and the building experience is absolutely incredible. For us though, the lightsaber-building experience will wait another few years until our son is just a bit older!
Thanks again for your question, Richard! I hope you have a wonderful visit to the Walt Disney World Resort!
Have a Magical Day,
Greg