I would add that you know your child best, and so you can probably determine if they'll be scared on the haunted house, pirates, etc. But having said that, we took our 2 yr old triplets, and they loved it! Some people said that we were silly taking them, that they wouldn't enjoy it or remember it. Well, we've got great pictures and videos of our kids hugging Mickey and pals! And they did enjoy the rides. They even cried getting off of the haunted house, because they "didn't want to get out of the buggies!" But we took them back when they were 4, and it was a totally different experience, and they did not remember much of the first trip. But we still have those memories!
But to directly answer your question, you'll be able to carry your 18 mo old onto almost all of the rides; just not the fast and big ones!
Enjoy your trip!
Wade