Hi Roberta,
Welcome to the Moms Panel! You're a smart Grandmother to plan ahead for your trip to Disney World with Grandchildren. It is a whole new world when traveling with little ones for sure.
Your 6 month old & 3 year old grandchildren can ride any of the attractions where they meet the height requirements (HRs) for attractions. You can find a list of the attractions with HRs by visiting the
Traveling with Little Ones section on the Disney World Website. There you will find a PDF file on the right side of that page that lists attractions at all of the theme parks that have HRs.
Some attractions don't have HRs. You know your Grandchildren better than anyone, but keep in mind that just because an attraction doesn't have a HR, doesn't mean that every child should ride it. Haunted Mansion is a great example of this - there is no HR but its dark & once you're on, there is no great way to get off of this 7 minute & 30 second ride.
Bring something to help keep them entertained in line - favorite small toys, snack, bubbles, etc. Be sure to give them breaks from their strollers. Try (it will be hard) to stick to nap times. Take advantage of the Baby Care Centers located in each theme park. Click
here for some tips on strollers.
Let me know if you have any questions about any specific attractions - I'm super happy to assist you. Wishing you many magical memories :-) Allison