Hi Jennifer,
I am super excited for your first Disney family vacation! I know that you'll create some amazing memories.
Your 3 year old and 8 month old can ride any of the attractions where they meet the height requirements. You can find a list of the attractions with height requirements by visiting the
Traveling with Little Ones section on the WDW website. There you will find a PDF file on the right side of that page that lists attractions at all 4 of the theme parks that have height requirements.
You know your kids better than anyone but keep in mind that just because children meet the height requirements doesn't mean that ride is appropriate for all children. Haunted Mansion is a good example of this. Its pretty dark inside and once you're on, there is no easy way out for a child that becomes frightened. Your 8 month old may do fine because most haven't developed certain fears at that age and your 3 year old may be afraid. This may hold true for character meet and greets as well. Most 8 month olds are very curious about Mickey but he can be a little frightening to some 3 year olds when its their time to approach for pictures.
If you have any questions about any specific attractions, I am happy to help you. I've also attached a question regarding a few tips for traveling with the littlest of mouseketeers - click
here.
Wishing you many magical memories :-) Allison