Hi Miranda!
That's sounds like a wonderful Disney vacation. You will be able to enjoy a lot of the attractions in the Magic Kingdom with both of your children. Most of the attractions at Walt Disney World, and especially in the Magic Kingdom, were developed with the whole family in mind.
Must things to do include:
- Dumbo
- Peter Pan
- it's a small world
- Jungle Cruise
- Carpets of Aladdin
- Pirates of the Caribbean
- Monsters Inc Laugh Floor
- Tomorrowland People Mover
- Winnie the Pooh
- Mickey's PhilharMagic
The Haunted Mansion is a fabulous attraction and one of my favorites, but some young children don't enjoy it. It's not really scary, but just the name "Haunted" gives children the creeps before they even go in. I recommend trying to do it. If you have another adult with you and can switch off with the younger child, then your son may also enjoy Big Thunder Mountain Railroad and Splash Mountain. Some young children enjoy them, but some don't!
By the way, since you said this is your fist time, I really recommend getting the book "The Complete Walt Disney World' by Julie and Mike Neal. It is a great book for first time visitors, because it has a lot of information and tips but is not overwhelming. It is also a beautiful book full of color photographs. It will give you a good feel of everything there is to do in the Magic Kingdom on your one day there.
Enjoy planning your fabulous Disney vacation, and write back if you have any more questions!