That's a tough call. All of the parks are wonderful, but if you have to skip ONE with a small child, I'd say Epcot. I don't think you will though, having 9 days in early October. I think you will make it to all of them.
I went with my niece when she was 5 and she liked Epcot the least. She did like the Chinese child acrobats at the China pavilion in the WS and Journey Into Imagination but that was it. Now she's 10 and really into science and it's her favorite.
MK and AK are must sees for most every child. MK I don't think needs and explanation. Everything there, with few exceptions is kid appropriate. AK is like a big magical zoo and I have yet to meet a 5 yo who doesn't like animals. However, AK can often be done in less than a day.
MGM has a lot of bright, colorful things that a child would love. Some of the rides are a bit intense for a kid that young (obviously ToT and such, but also, my niece was terrified of the Great Movie Ride because of certain scenes). Some might say to skip MGM if you need to skip something, but keep in mind, there are tons of non intense, interactive things here for children.
Epcot is MY favorite park. Always has been since my first trip to WDW when I was 8 years old. The attractions are science, technology and history based. The WS is my absolute favorite thing at WDW because I love feeling like I'm in a far off land. However, it may be kind of boring to a child that young. My niece was bored there at 5. However, I know it varies from child to child. If they really like the sort of things Epcot offers, they might love it.
I would at least stop by Epcot for a little while, and do Journey into Imagination. My niece liked that.
But really, I think you will have plenty of time with 9 days.
Hope this helps!
