I don't believe a lack of appreciation for the amount of work involved is the issue. I believe it is a lack of manners. Even if you had given them "only" a blanket for a gift, would that not have deserved a thank you as well?
Something for us to remember as we raise the next generation. If we, as parents and family, don't teach basic manners, they won't have any. I never give a gift for the purpose of getting a thank you, but I have to admit it is pretty disappointing and even somewhat irritating when I don't get one! (Honesty at its' best!)