While you only now what is best for your family, are you sure you could do it? Leave her home, that is? I'm sure you will miss her terribly, and she would miss you too! Would you get there and constantly be doing the "she would have really enjoyed this ride/show, etc" thing? I know that I couldn't do it, but only you know whether you can handle that.
Also, what would you tell your 18 month old about where you are going without her? I have an 18 month old DD right now and she has been in the "me too" stage for at least two months. She is well aware of what her older brother and sister are doing and she wants to be right along side them. At 18 months old, they are old enough to understand much more than we usually give them credit for (even though they usually can't express too much yet)! Would she feel left out? My 18 month old is even helping us take the links off our "countdown to disney" paper chain. She makes sure that she gets a turn with everything by pointing to herself while whining a cute little "toooooooooo". How would her siblings feel not having her there? My older two would not want the little one left out of the fun.
If it is the drive down or possible fear of characters that is making you leary, there are many suggestions that people post on the boards to make things easier. I wouldn't let that be the deciding factor. We have driven from NJ 5 times since my first child was born with varying ages (they are currently 6 yr, 4 yr, and 18 mths) and they have done fine in the car because they knew we were heading to disney.
Like I said before, only you can decide what is right for your family, just be sure you have thought out all the pros and cons first.
ETA: OOPS, so sorry, op! I thought I had read that your 18 month old was a girl. I just re-read your post and realized that he is a boy. I apologize for turning him into a girl in my response, lol! Must be because my little one is a girl

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