Just be sure to be super excited, big smiles for the characters, at that age they definitely vibe off of you. But like someone said, all little ones are different. Both of mine loved the characters and we had some great moments we will cherish and remember always. My oldest was 14 months her first trip, the next little monkey was 11 months. Our most recent trip with the newest little one was our 7 month baby girl. She was hysterical, and loved the characters too.
The funny thing is that on my oldest's 2nd trip she was 3 and hid under the table at the our first Chef Mickey's meal. By the 3rd character meal she was fine.
I don't think it hurts to get them familiar with the characters, Mickey Mouse Clubhouse, stuffed animals. My middle guy's first word was "Mickey", so he was pumped to see him.
As far as rides they are pretty easy that little because they don't quite grasp the scary concepts yet. We had one that got a little upset in the Haunted Mansion because of the stretch of darkness but otherwise handled it great. Pooh Bear, Small World, Nemo are all great....lots for them to look at. But you can still pul off Pirates, Jungle Cruise, Snow White, etc.....the ones that they might be too scary their next trip.
I get longwinded on trip advice with little ones, sorry! Feel free to send any questions my way, happy to help!