A denim skirt will be fine...especially with a nice top.
For young kids, anything goes. For "middle" kids...these are the ones who dress up. For teens....I read DCL's dress requests to DD and tell her that she must dress appropriately. She really likes the dress up bit, expecially since none of her "buds" from home can see her and think she's uncool.
My DD's (age 18 and 21 now) and I (old enuf, lol) have all worn denim skirts to dinner. I would not do so on a formal night, but I had no problem wearing them the other nights, if so desired.
For casual night, a denim skirt is a highly practical choice. Since these skirts are easy to dress up or down, your daughters will probably be able to wear it again the next day -- I'd probably squeeze two afternoons' wear out of it.
GET UP TO A $1000 SHIPBOARD CREDIT AND AN EXCLUSIVE GIFT!
If you make your Disney Cruise Line reservation with Dreams Unlimited Travel you’ll receive these incredible shipboard credits to spend on your cruise!