Hi, M.. Well, when my son was around that age, we used to hire a clown, who did about an hour or two of keeping the kids happy and busy. Different clowns do different kinds of things. A magic clown would be good for older kids, but 2-year-olds don't usually have the attention span for that. Try getting a clown who does balloons, songs, etc.
I love having parties in parks, but that depends on your resources. You might think about having your home as a backup, because here in Florida, October is still in hurricane season, and we could still get rained out. Try to get a couple of helpers to help you serve the kids, keep them entertained, clean up, etc.
A Halloween theme is wonderful. You can get all kinds of cupcakes, goodies, and a themed cake without having to special order any of it. Just don't do anything too scary or allow any really scary costumes -- 2 year olds get nightmares really easily, and it takes them a long time to get over them!
I hope you have a wonderful time!
Peace,
Syl