I am not 100% sure but am planning on dressing up. I am still working on a costume for DW and DD (1 year).
I think I will be Robin Hood and DD will be Tinkerbell. Looking for a Maid Marion costume for DW but no luck so far. Alternatively, all three of us might be pirates as a dry run for our cruise on the Wonder next year.
A big challenge is finding something that is comfortable for DD. We tried the Cinderella costume someone gave us but she doesn't like the frills.
Since WDW makes you feel like a kid, whynot dress up.