We did matching t-shirts last year. Even poor DH relented to wearing the same t-shirt one day !
Have to say, as a larger family, we did get a lot of comments on the t-shirts - very positive ones, particularly from CMs, who said it made it so much easier when counting us through on rides / at the entry gate, etc.
The kids and I all loved having the matching shirts, and I'm hoping that DD1, who will be 13 on our trip next year, will still be happy enough to wear them again.
I made these t-shirts with the kids before we left - tie dyed, with mouse ears appliqued in the material of their choice. So easy to pick my kids in a crowd !

(Incase it's not obvious, I fully immerse myself in my inner kid - I really couldn't care less what others think. We all had a ball.)
Cheers,
Sally