It depends on how anal the elders are in the hall, and how easily some can be "stumbled." We used to have dress up parties in the hall, but the elders put an end to them...even for the kids! "Decade" parties were "glorifying an era" , and "tacky parties" could be mean to people who dress tacky.
One sister successfully had a "Bible" party where kids could dress up as Bible characters and play Bible games.
Good luck! When I was young I went to lots of JW dress-up parties (in the summer) and they were fun.