Hi Cassie,
A first family vacation to Walt Disney World with your young children will be so much fun. I believe that each Walt Disney World vacation is unique to the family taking the vacation. I can give you some suggestions based on your children's ages; however, you will need to gauge your children and family specifically to consider what they will most enjoy. I have a couple of suggestions for doing this.
First, order the
free Walt Disney World planning DVD. Have your whole family watch it together! Your older two children can get an idea of some of the attractions that they are interested in experiencing. Next, let each of your older children create
customized maps of each of the Disney theme parks. In doing so, they can indicate the attractions that they are interested in. These maps will be delivered right to your house. I suggest that you consider their ideas, and then let each member of your family choose two or three attractions that are their "must do" attractions for each theme park. Make your plan to experience those attractions, and then anything else that you get to is a bonus!
With your two older children, they might be ready for some of the slightly more thrilling attractions at Walt Disney World, such as Big Thunder Mountain Railroad Magic Kingdom. Just be sure to check height requirements for any ride such as this. Your whole family might enjoy character meals such at Chef Mickey's if they are interested in meeting characters. Your littlest one might enjoy running around some of the play areas such as the Honey I Shrunk the Kids Movie Set Adventure at Disney's Hollywood Studios.
I would love to help you plan more Cassie; however, I am limited in the amount of space that I have to answer. Please do not hesitate to ask if you have any further questions!