when my DD was just about 17 months old, we went on 2 cruises, nearly back-to-back. The first was a 3-nighter with family... on the first night, she was perfect and sat still and entertained herself through the entire meal (sorting out the sugar packets, eating the rolls, playing with silverware, etc). The second night, she started getting antsy around dessert time and wanted someone to climb up and down the stairs with her. By the third night, she was ready to go after we got our appetizer.

it wasn't too bad, though, as we had family to rotate stair-climbing rotation with.
the second cruise was a 5-nighter, which we had to fly to the port of embarkation. we flew up morning of, assuming she would get some extra rest on one of the car rides to/from the airport, or even on the plane. WRONG! so we woke our little darling up at 5:30am to start the day, and she refused to sleep after that (too much stimulation). This set the tone for the rest of that cruise, and we were only able to eat in the dining room on one or two occasions. This was not a particularly kid-friendly line (no babysitting, and i think only a handful of little ones on that sailing), so they weren't too good at hurrying things along either. After the first 2 nights of trying to eat at a multi-course sit-down meal, we ended up just opting for the casual buffet or room service.
We are cruising on the Wonder shortly, and DD will be 20 months. We are also traveling with other kids her age, so I'm hoping they'll entertain each other at the meals. We are definitely going to attempt the dining room all 3 nights, and one of those nights we are hoping to try her out in Flounder's Reef and go to Palo's alone!
My advice is that you make sure your child is well-rested before boarding, and bring along some things to keep her occupied in the dining room. Definitely do your best to make it to all dining room meals. Your DD might just surprise you. Also, the staff is generally very friendly to little ones. On one cruise, some staff saw my DD getting really fidgety during dinner, so he crafted some little origami paper frogs that could leap around the table! Most of the staff (on my previous cruises at least) have children at home, so they understand. And generally, they try to help you out also.
Hope this helps. This is our DD's 4th cruise coming up (first on
DCL), so I will be happy to answer any other questions you might have about cruising w/ little ones.