we asked for and got substitutions on the kids meals a couple times. DD5 had the roasted chicken legs at Whispering Canyon but won't eat mashed potatoes. they subbed buttered pasta at our request. Same thing for DD7 at Akershus - she got the grilled chicken with buttered pasta instead of mashed potatoes.
DH and I did not ask for any changes, but after reading how the chefs go above and beyond to accomodate special diet requests and allergies, I can't imagine they would even flinch at subbing one side for another.