Hi Janet,
Thank you so much for visiting the Disney Parks Moms Panel with your question. What a special trip you are planning - to visit Walt Disney World with your grandson! Let me offer some suggestions for your trip.
First, I suggest that you and your grandson both watch the
Walt Disney World planning video online. This will start to give you some ideas of the theme parks that are most appealing to the both of you. Depending on your specific interests, there might be certain parks that appeal to you both.
If I was choosing based simply on knowing that you are going with your grandson, I would suggest one day at Magic Kingdom and one day at Disney's Hollywood Studios. I suggest
Magic Kingdom because it is where the magic began in Florida, and has so many great attractions such as Big Thunder Mountain Railroad and Space Mountain. At
Disney's Hollywood Studios, you will find great attractions such as Toy Story Mania!. Disney's Hollywood Studios is also the location for all things Star Wars, including the great interactive area Star Wars Launch Bay.
As to the park hopper option, since you mention that you are only going to Walt Disney World for two days, if you only find that there are two parks you want to visit, I think that you will find a lot to do simply visiting one theme park each day, and therefore would not need the park hopper option. If you decide that you want to add Epcot or Disney's Animal Kingdom Park to your touring as well as Magic Kingdom and Disney's Hollywood Studios, then you would need the park hopper option.
I would love to help you plan more Janet; however, I am limited in the space that I have to answer. Please do write back if you have any further questions!