My son was 15 months on our first trip and he LOVED the whole thing!!!
We were prepared for him (and my then-3 1/2 YO) to be terrified of the characters, but they both absolutely adored them. We took a sling (that we borrowed) for the baby and every time we got on a ride, he went in the sling. He slept there a bunch, too (Haunted Mansion).
We dined mostly at buffets and all-you-can- eat places because there was a huge variety of food and they had no problem bringing him a plate and a drink. I can't remember if we were charged for his milk or not, but if we were, it was no big deal. All the food was free. Well, not free, we had to dine there, too, but they didn't charge us for the baby's plate.
Matthew rode every ride that we rode that didn't have a height restriction. He loved Dumbo, Pirates, TTA, Kili Safari, Maelstrom, the Mexico ride (I can't remember it's new name), Great Movie Ride. Whatever! We parent swapped a bunch, too. There's always something for little ones to do near the big rides. It was a great trip and really pretty easy.
Have a great trip!
--H